Are you sure he is not splitting in two (aka cloning)? My BTA hides when he is about ready to split. The two ends will basically just go in opposite direction, tearing the center section. The torn section looks kind of rotten I guess. The nem is usually pretty wilted (sp?) during the process. You might just give him some shade since you uncovered him but keep an eye on him.

BTA will do fine under PCs.