Hmmm, if they are more slug-like, they could be a species of flatworm. If they have legs, they are probably some type of amphipod/copepod. It's also possible they could be nudibranchs, but that seems less likely than the other two options.
From the pictures, we can't really tell what sort of life they are. Can you see them well enough to make out whether they have legs, a head and an exoskeleton, or their bodies are completely soft?