Perhaps you could be more specific... what does this 'stuff' look like? Specks? cotton? What color is it? Where is it at?
Is it that white spot near its tail? I can't tell whether thats some sort of speck on the glass, or the problem. If it's the problem, it looks to be fungal, but could also be some sort of bacterial infection from a wound.
"What, exactly, is the internet? Basically it is a global network exchanging digitized data in such a way that any computer, anywhere, that is equipped with a device called a 'modem', can make a noise like a duck choking on a kazoo." - Dave Barry