Mine did this a few times. I think it is growing when it does this. It would "shed" a layer off like Sammy said.
I cut the top off mine with scissors last week, then cut the top in half. I gave one part to my buddy, and now have two in my tank. They are really cool corals. Mine seems to do really well in bright light / good water flow. Polyps fully extended and healthy. I've also read they need a good supply of calcium to remain healthy. 400-450ppm.
[ August 31, 2001: Message edited by: mal ]