Blue Spot Puffer? Algae?

[fishoholic]

Puffers will eat snails and shrimp, but usually leave corals alone. If it's a toby puffer you're thinking of they are known to nip the fins of tank mates.

[hillegom]

If you use RO water, you will have less problems with algae. As well, feed less.
How old and what size is the tank? How much nitrates do you have?