My Lemonpeel nipped the corals for a month or so then it quit and now just pecks as the live rock and leaves the corals completely alone. I found that when i added the Coral Beauty (slightly bigger), that he calmed down a lot and was less aggressive and thats when the coral nipping stopped.
The Lemonpeel hated the Coral Beauty at first, went to nip it (until he realised the Beauty was bigger, then he quickly darted away). Then he avoided it, but now they are best friends and swim together around the tank.
I guess it all down to fish personalities but i did find that introducing the larger angel made him much more passive.
Hope this helps