Thanks! I looked up sole more pics and that's exactly what it is. I saw that they're herbivores and it seems he ate my clump of maidens hair. Is it reef safe though, or will it bother coral?
If you have Maidens hair it will not leave it. So, it will be fine. Without the Maidens hair, i'm not sure. My guess, without the Maidens hair it will likely die.
Maidens hair (Turtle weed) needs plenty of oxygen and flow. If one or both of these factors are low, that could be the reason why it's gone. I doubt the crab would have completely ate it's only food supply.