Great Race of Yith - Speculation

If the Yithians are long-living, unmoving, and aquatic, then wouldn't they look much more akin to sea anemones or the scallops? Instead of pea-green leathery skin and big red eyes, wouldn't they have clearish-blue or orange flesh, and beady little blue eyes, with fronds of filter and feeler tendrils to help move debris away or towards the central body?

The great claws have a sort of "foreskin" to protect them when pulled in during times of boredom or fear. The claws are shaped like a squid beak or claws of a soft sea floor animal, with one movable clawed digit and the other just a firm "case" for it to grate against and slide into and against.
The eyes are beady and dark like the eyes of a scallop or snail. Three eyes are clustered on the end of a stalk, allowing for 360° vision. The stalk is covered in a thin clear membrane filled with fluid that cleans the eye lenses and circulates nutrients to the stalk while allowing some degree of protection against anything pushing against it, like a waterballoon cushion.
The shell is like that of a conch, very tough, constantly growing, incredibly heavy, and dense. For a sea creature of a Yithian's size, the shell is too heavy to move even for a conch bigger than a man.
The nostrils or three-nasal tube is said in the books to have a unknown purpose except to the Yithians. But based on what they are, and how they are, these tubes are probably for water circulation, air circulation, water-spouting, and possibly chemical detection. Like most unmoving large sea floor creatures, these tubes are probably related to feeding and helping to eat microscopic creatures and plants in water or very soupy and dense air.
The excess of anemone-like tendrils help to move food particles toward the body or tubes, or to help move things away from the body, and filter goodies from the water. These may be loaded with stinging cells and only fellow Yithians and creatures with thick mucus coverings never notice them. Either this, or they are like simply little helping arms of creatures with unstinging anemone-like arms.

With this kind of anatomy, the Yithian's flat slab books are probably held in one claw and at a angle where a pulled in eye stalk can just stare up or upwards-and-at-a-angle at it without having to bend or curve. Their writing would then be a mix of high-stimulus dollops and whirls with certain areas of a page having scent and chemical smears on it, so the sensory organs and not just vision could be used to read, since vision is not a major sense in unmoving sea creatures.