I have had experience in different types of mud/qs, and there are varying characteristics.
For organic mud (from moss and decayed plants), it can vary in texture, depending on what type of plant has decayed. While plants such as moss can form more coarse muck (like soggy cereal), mud from other decayed plants such as algae, grass, and water lily leaves/stems (not rhizomes) can be rather fine textured, and stickier when at certain thicknesses. In organic mud, your buoyancy point is around shoulder to neck deep, with it being neck or chin deep for really loose or water-logged mud. But, with organic mud, you can easily pull yourself under, but once under, no matter how much you move, you will not rise back up to the surface. You will stay at whatever level you pulled yourself down to, and will have to physically work or climb your way back to the surface. For finer textured organic mud, it can be rather sticky when it is at a certain thickness, and when even thicker, it becomes doughy, and separates easily from skin, turning itself inside-out as you pull yourself out of the mud. However, there can be situations where organic mud can be on top of water below. Moss can grow over top deep water, forming a mat, with live stuff on top and decayed stuff below, with water below that. But, there could be open areas where the decayed, peaty mud is exposed, and can be treacherous, because once your feet break through the bottom of that layer of mud and start dangling in the water below, there is nothing to support you, and you could sink all the way under, unless you can manage to get your body on top of the mud and solid vegetation.
Then there is the mineral-based mud (quicksand/clay/silt). Because they are mineral-based, they are heavier, and therefore you don't sink so deep before reaching your buoyancy point. It varies by material, and by water content.
For quicksand, if it is quite thick, you may only be able to sink to your waist before stopping. But, if it is loose and watery below, it may be possible for you to sink to your shoulders, especially if there is a water pocket or underground stream below. It is not really sticky, but does lock up with large movements, while small agitating movements allow for movement through the liquified sand. It can be rather difficult to sink really deep in quicksand, especially thick stuff. When you do manage to force yourself past your buoyancy point, any movements will cause you to rise back up to your buoyancy point, unless you stay at that deeper point for too long and it settles and packs around you, locking you in solid sand with a huge digging job to do that could take a very long time, perhaps even an hour or more (or you may need external help), and may have to endure painful leg cramps at the same time!
Quicksilt is more like a finer-textured quicksand. It also is quite dense, making it difficult to force yourself past your buoyancy point, which is similar to that of quicksand (waist deep when thicker to chest deep, if watery enough). It also locks up if you move hard, but flows with agitating movements. Because of its finer texture, it can be stickier than quicksand, but some kinds of quicksilt can settle quickly, and pack hard, such as in Alaskan mudflats. It is a lot of work to try and force yourself to your chest in quicksilt, and I hurt one of my shoulders years ago forcing myself shoulder deep. Just like with quicksand, if you manage to force yourself past your buoyancy point, then any movement will make you rise back up again.
Finally, there is clay, whose properties can vary, depending on water content, type of clay, and if there is any presence of electrolytes (such as salt). Clay, while dense like silt, differs in that it expands in water (for a visual idea of clay's microscopic characteristics, vermiculite is actually a clay), which then lowers its density, allowing for a deeper buoyancy point, and a bit more ease in forcing yourself deeper, unless it is not thick enough (you need some thickness to push against to force yourself deeper). But, when clay is too loose, you will bob back up to your buoyancy point when you move, or if it is loose enough, even when you don't move. When it is thicker, it gets stickier, allowing you to more easily pull yourself down deeper, or under, but it can also hold you down, just as Sinkman61 always talks about. So, you then have to work yourself back to the surface. Some types of clay, such as Bentonite, can expand a lot, with a nice thick texture even with more water content, lowering its density, and is super sticky. So, it is even easier to sink deep and pull yourself under in Bentonite than other clays. But, for other clays, if you add salt to the water, it changes the characteristics of the clay substantially! A liquid clay slurry, with salt added, becomes a sticky peanut butter! So, clay at saltwater mudflats can have a relatively low density compared to non-saltwater clays of similar thickness, allowing for a deeper sink before reaching your buoyancy point, but its stickiness could also hold you down, and struggling movements can work yourself deeper, with the stickiness holding you in. Having footwear can make matters worse, since it adds suction to pull you down more, which is really bad for extremely thick stuff. With footwear, attempting to lift one foot up may have little movement, while it forces the other foot deeper, and then trying to lift up the deeper foot has little movement but forces the other foot deeper, and so on and so on!
I hope this helps.