Here's an honest answer. Unknown. They can be, and yet they also could be a potential nibbler on various things. No angel is ever perfectly reef safe (uh oh, gonna get flamed now for sure!). It varies from individual to individual, with some species being less likely than others. In general, the pygmies (afrcian flame-back is one) are thought to be more reef safe than others. I have a flame angel in my 75, and the live rock is covered in many areas with tunicates/sea squirts, which apparently are irrisistable treats for angels. Well, could've fooled me, he hasn't even touched them. However, he does have a taste for some hair algae, and occasionally picks at some other leafy algae growing. Doesn't mess with the fanworm population either. Its your risk to take, I don't think that you shouldn't, but be ready to remove if necessary.