Mine did the same thing, and quickly! However, it stopped about an inch from the edge. (turned white from center out)
Now it starting to actually grow back. In the last 3 weeks or so it has grown back about a 1/4".
What I did was, took it it out of my tank and took it over to the sink. Then blasted it with tank water using a turkey baster. I put it back in and hoped for the best. After a day I noticed that it stopped, and a day later the flesh started growing back.
Never did figure out what caused it. I've heard Monti's do this sometimes. I plan to get another powerhead soon. The only thing I could see as a culprit was a yellow watchman goby I bought that perched on it. Within a day or two is when I noticed the white spots. He was nipping at the flesh that was coming off. The goby ended up getting to close to my powerhead and was stuck to it. After that the Monti started coming back.
So I have no idea what happened or what actually caused or fixed it.
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