How old is your tank? I started mine in February and I just started putting my tangs in a month ago. They are always picking on my rock, but have yet to touch my corals. I was actually going to clean off my loc lines because some algea had started to grow on them, but it's now half gone because the Sailfin and Hippo are eating it.
This is why you usually find that it's better to put Tangs in a more established tank, your sure to have some type of algea for them to munch on. Oh, and my Hippo Tang even likes to eat the red slime algea.
Happy to report I don't have much more of that though.