This is an idea that first was suggested after SpectrumGirl shared her experience with quicksand in the chatbox: quicksand slime girls. Or, to the use proper Felaryan terminology...
Sand Slimoids
Category: Slimoid subspecies
Where normal slimoids are transparent and smooth, sand slimoids are…well, sandy. Sand slimoids differ both inside and outside from their more aqueous cousins; their internal plasm is heavily saturated with sand, and their skin membrane is incredibly sticky, but almost always covered in a layer of sand. The internal sand is an adaptation that gives them a much greater density than most slimoids, which, when combined with their sticky skin, allows them to ensnare prey with ease and keep it trapped. Combined with their natural camouflage, these traits mean that sand slimoids excel at ambushes–an aptitude which is useful, as sand slimoids are nowhere near as fast as other varieties; their increased density comes not at a gain, but a loss of plasm density, making them relatively weak when doing anything other than trying to hold something in place. Despite their abilities, sand slimoids are usually harmless to anything larger than an insect; they can, and sometimes do, eat creatures by absorbing them into themselves and using their interior sand like millions of tiny gallstones to grind them up, but the process is slow, even more than normal slimoids, and the concentrated extra mass makes it near-impossible for them to move. Beach-dwelling sand slimoids instead prefer to eat via filtration, running water through their bodies and digesting plankton and algae. Desert-dwelling sand slimoids aren’t blessed with such easy food, however, and often have to live off what few creatures they can ensnare; additionally, their skin needs to generally be thicker and much less permeable to keep moisture inside. Alternately, in areas of heavy vegetation, sand slimoids can use their body as an abrasive to grind off plant matter for digestion.
Sand slimoids have few natural predators, mostly because few things want to eat that much sand. Apart from the lack of nutrition, a sand slimoid is generally at least twice the mass of a similarly-sized creature, can be very rough on the digestive tract, and like all slimoids, are quite malleable, slipping through the fingers of a would-be predator much like the sand that makes up the majority of their mass. This means that most sand slimoids are very old. As a sand slimoid ages, they increase in volume, but those that take humanoid shapes tend to stay keep their apparent size about the same, with the extra mass hiding in the ground below their feet. The most ancient and largest of these can be over a hundred meters in diameter, and have been known to feed themselves by using a humanoid shape as bait, drawing a predator close before snaring them and drawing them deep within themselves. Older sand slimoids also have more developed and complex personalities, and their generally slow pace of life means they learn to content themselves. Some, especially the beach-dwelling type, have become more social, and as they don’t need to eat humans, can develop good relations with nearby communities. A few costal towns have even bargained with larger sand slimoids, trading them ground food in exchange for protection against predators.
Sand slimoids are most often found near beaches, the borders of deserts, or oasises, where they have a plentiful supply of both water and sand. They are sometimes mistaken for earth elementals, especially when dwelling in areas with dirtier areas. They try and avoid these areas, however; large clods of dirt or pebbles can result in weak areas due to the lack of plasm.
Subject: Sand Slimoids 
Fri Nov 13, 2015 2:07 pm
Having them appearing a certain size and hiding the rest under of themselves is a good trait that make sense. And for those who live in the Akaptor desert, they could feed on Shimmerlings I think ?
