As adinfinitum said, what you have observed on the corals probably sounds like limpets cleaning up dead or dying tissue. It is hard to tell, but they are likely not the cause of your problem, likely just a side-effect of what has occurred. There are also some white oval shaped spots on the braches that appear dead - these may be due to feather dusters living in the coral, which is pretty common, especially in mari-cultured pieces (which I'm guessing this is judging from your photos but I could be wrong).
So, in my estimation it is something else. Pattern of tissue loss isn't consistent with AEFW so I don't think that is it. How long has this being going on, and how many pieces are effected (is it just this one or is it more than one)? It looks like you have coralline encrusting on the dead base of the coral, so looks like it has been going on for a bit of time anyway? I wouldn't rule out some type of predatory crab or worm - if so, viewing at night may be your best bet. It also could be water chemistry related - can you provide more info - any issues with alk swings, are you running carbon and gfo and have you changed those levels, any changes to salt type, etc?
