Poor little jelly, it never had a chance, another one was right there to get him if it got away.
Were they were not compatible to be in the same tank?...what a shame.
Maybe the jelly was not going to live, and the stars were just cleaning up before an actual inevitable death
I wouldn't have put them together. Also consider that, besides the fact that many LFS don't care well for some animals that are dying, the green brittlestar may not have been fed for awhile.