Sorry. Very hard to get pictures. However for about the last couple days he’s been sharing his hidey hole with a small 2-3” Brittle star. Is this normal? He also has not been out nearly as often after the star moved in, could it be he’s trying to protect his territory?
Probably not. Green brittle stars can sometimes be fish eaters, but the rest are benign.
And hard to tell from the pic, but yours might be a serpent star, which are very benign.