Help with starfish!

I got a little red starfish a couple months ago and it has been doing great in my tank. But this morning when I woke up, its leg looked cut and was almost completely white! Did something attack it?

 

starfish get this often in captivity, no one knows the cause because often times even completely healthy starfish get it. at this point i would suggest cutting off the leg to rid the starfish of the disease before it gets a chance to spread up to the body. thats where itw ill do the real damage and most likely result in the starfish's death. as for the leg, in time it will grow back.