4 things come to mind when I look at this pic.
1. I've seen a similar sebae event, but don't remember where, I'll look around though.
2. I do not recall any post here or anywhere else that reports of a sebae splitting in captivity, but it must be possible.
3. I wonder if the crab in the anemone is simply causing it to contort in a strange way.
4. That sebae is really bleached, but this doesn't seem to be uncommon with this speices. When collected they seem to loose there zooxanthellae really easy. This is a rather moderatly difficult speices to try to keep, I would also suggest MH lighting. Broomer5 had one in a post where he showed the progress of it going from white(unhealthy) to a dark tan(Healthy). It took months
Of all the events with anemone splitting that has happend in my tank with BTA's has occured only at night with lights off.
You should know by moring if it was a splitting event.
Good luck
Thomas