Starfish Pilates , it might catch on .
Like OgreMkV said it's releasing sperm or eggs to breed, that's what the smoke is, a lot like corals really. Not sure why it has to arch, maybe so the 'smoke' gets caught by the current better.
There are no other starfish in the tank so it's efforts were wasted .
Max size is about 12cm (5 inches), although their growth rate really slows down at 4 inches. Excellent community fish, except around small species (eg neon tetras).