Ya that crab in the picture seems to have made that his home. Its a small branched coral right up against the front of the tank so where I can watch him all day. Still trying to figure out what that Red thing above him his. I think its coral. Bc depending on the light it gets reflective and glows. Also, my turbo snails love the red algae. I watched one put its entire mouth around one of those "sacs" lol and devour it.