tang problems

[nil bob]

sorry, tried to take one with a better camera but the tang is sleeping in a crevasse

[nil bob]

got more pics.
there is a black brittle star sharing a hiding hole with it. would it be a suspect to the damage? nothing unusual about water parameters. only tank mates are two saddle clowns some small hermits, some herbivorous snails and zoanthid polyps 101g cube with 13g sump