Look for monti eating nudibranchs also. They seem to eat the tissue from the bottom up. They are white, they look like fuzzy cotton like threads they're pretty small, and will eat all your monti's. They seem to go only for monti's at least to start with.
There is no cure, just manual removal and maybe removing all your montis if they get out of control.
You can search for pics, but don't rule that out especially if you've added any monti's in the last few months, they can have eggs with them and hatch eventually even if you didn't see them when you bought the colony.