Found this guy hanging a ride on my hermit crab. I tried to identify something like this previously in a different part of the tank and now appears to be attached to the crab shell (and a void in its previous home). It has a black "tentacle" that searches around for food. He has doubled in size since being on the crab and the white part you see in the photo appears to be new growth. The crab doesn't seem to mind carrying this thing around and allows it to graze all over the tank.