As far as I know, RTN (haven't heard STN before, but fits) should only affect the tissue, not the skeleton. Have you checked pH? I've never heard of acidification problems causing that in a tank.
Possibly, the RTN is happening and the Blenny is breaking off pieces trying to eat the tissue, but not actually doing the killing. When I had RTN in my tank, the affected corals were swarmed with hermits.
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