Go buy some ascorbic acid from the store and put it on the MB stain. Comes out, just get it slightly wet and let it sit awhile. I think I used peroxide too.
I had a bottle from a fish store that had some crusty MB on the top of the bottle. I didn't think anything of it when the MB flake fell on the counter. Later when it was exposed to water (didn't immediately see it), I figured out my mistake.