You may, in fact, be interested in seeing a series of pictures of my brittlestar (the same species - Ophiomastix annulosa). Often times, these injuries
to the disk can be related simply to eating too big a meal, and the food punches right through. In
acclimation shock I would expect to see some sign of disintegration of the arms.
Note the time frame - 9 days - simply astonishing!
Another:
Brian is absolutely right

The thing to do, simply, is keep params ideal. This is the best thing for it, in addition to removing any critters that might be a pest (eg hermits). Or consider isolating the star in the tank in a container (such as a critter keeper). But ideally the star would just hide at this point.
It is especially important that
specific gravity remain at 1.025-1.026.
It is very unlikely it is a disease. In fact, brittlestars have a fascinating assemblage of symbiotic bacteria that can fight off all sorts of mysterious things (and are being studied for our use, too).
However, if the arms do start falling off, that is a bad sign. I don't give up hope real easily, but it isn't the best thing. That being said I've seen stars in very bad shape that have recovered nicely.