This blob of gray stuff has suddenly appeared in my 10 gallon aquarium where my kids' comet goldfish has been happily living for 6 months. I've also started to see some green algae form along the glass and filter intake. I'm considering adding a few snails to eat the green stuff, but this gray blob has me spooked!
I will only have to guess, but I think it is a biofilm; most of the surfaces in a tank are covered by a thin layer of bacteria.
When the bacteria layer comes loose it looks like the stuff you have in your tank. I do not think it does any harm in your tank.
If this is in a place with a lot of dust, it might be from the surface of the water too; no matter what it is, you might want to remove it from the tank.