I've had coral RTN and it didn't necessarily start from the bottom up Is the coral still like the picture or has it progressed?
I don't think its a damsel either...
Sometimes its just that one frag Sure you dont have redbugs or AEFW's right?
There was a Xanthid crab in my reef that ate all my zoanthids, mushrooms and then began stripping my SPS...took three weeks to catch it.