My first thought was salt, for something a creature might want to get rid of, that others still might want - and you wanted it red, so maybe iron, or other hard minerals. Partly inspired by the threads on a creature's fear of water caused by osmosis, to be fair.
It looks like the creature is living in fairly mineral rich environments - in between rocks, near the sea, and they don't move so must have some way of regulating themselves of minerals present in high concentration. Most creatures can tolerate a fairly narrow band of salinity, because osmosis will lead to too much or too little water pressure. Other minerals are also helpful in reasonable amounts, but dangerous if the amounts get too high.
If the creatures are living in areas where the mineral/salt content may vary, since they can't move they have to have some way of dealing with the excess - in this case, instead of regularly getting rid of them, it is expelled once it builds up to a certain point. Since the hard minerals are still good, other animals may benefit from them - which has an additional bonus of tending to carry them away from an area where the concentration is too high for your critter.
If the products are mixed with the creature's usual waste, that might be a problem for other animals using it, since it would also have the toxins and byproducts the critter needs to rid itself of and which other animals won't want. It may have evolved to expel different types of wastes separately, perhaps the red is specifically beneficial because it contains mainly concentrations of minerals and salts, and there is some other waste product(s) that contains the toxins, that animals can avoid. Of course, this adaption only makes sense if the creature benefited from the animals' presence somehow, maybe the increased traffic means more manure to grow the plants it eats, or attracts whatever its prey is.