Macnreef,
I had the exact same thing happen to 2 montipora caps and 2 M.digis. It happened in the matter of a week. I suspected nudis, but I never found any.
All my acros were unaffected. Also, my M.vietnamensis was unaffected. Good thing, 'cause it's my favorite.
But I lost every other monti very quickly...
Sorry to hijack,
E.