Well, in order to know, I would need more information about the set up, water parameters, how long they have had the brittle, and tank inhabitants. Also how often it is fed. Could be a feeding injury (see my site at
http://ophiuroid.home.att and go to Tabloids section and scroll to the bottom...also some pictures under the "basics" section).
Whatever the cause, the fact that the arms are intact is a good sign. The brittle SHOULD NOT BE MOVED to a different set up unless water quality is a real issue...if anything picks at it, it should be isolated in a container in the tank. And keep it well fed.