I checked it again today, it hasn't become any larger. So I removed what I could and turned the mesh bag a bit so that it is entirely underwater.
Clarification - the detail in the photo is the blue/greenish stuff across the middle of the photo, looks almost iridescent. I don't know how food would get up that high in the filter and not get swept out by the current, if that was the case. I did notice it was anchored to the the part of the mesh that was not totally submerged - I think if it had been entirely under the water, it would not have happened.
Also, the beneficial bacteria is green? Really? I've always thought green is a result of photosynthesis. Or gangrene.
So, how would you tell the difference between it and algae, unless it can be found in a dark area? The filters in my TopFin always end up being an ugly looking gray, never any green (always in a dark area).