Why is my tang doing this

My orange shoulder tang keeps picking at this rock along with one end of another piece of rock. There isn't any algae on this rock and there doesn't seem be any thing for the tang to eat on it, however it keeps biting off pieces of this LR and it keeps getting more and more white. Why is my fish doing this??? Here is a picture of the rock...

 

There's algae there, you just can't see it Your tang is grazing which is perfectly normal. They will peck at a mere film of algae so just because you can't see the algae, doesn't mean that it's not there

 

I agree with amcarrig she is just grazing and you can not see what she's eating, Try a nice seaweed sheet for her she's probably hungry.

 

I have been feeding a seaweed sheet every other day (however the last couple of times the wrasse and goat have been competing for it) There is also obvious algae growth on some of the other rocks (up to a 1/4" in a couple spots) and the tang does not even touch it...


Strange...

 

I had the same issue with my tangs but they also died within a couple days (bad store not due to my own negligence) how long have you had this tang?

 

It's been in this tank for about a year, although I have only had the tank for 2 months at my house