It is possible, but I rarely see my boxing crab and I have 11 brittlestars in a 45 g. Now and then though - typically after dark - I do see it.
Without knowing more about your tank and the situation, it is hard to know. Brittlestars are implicated, unfairly, in a lot of things. Personally, I think it is unlikely. If it is a green brittlestar, it is a possibility certainly - but what do you feed the brittlestar?