5 threads in 5 different forums all on the same topic, might be a new record.
Is this in a planted tank? Is it in a cichlid tank? Do you have DIY CO2? The reason it's there could be any number of things. However, just wipe it off with a toothbrush or sponge or something.
They make products to clear this stuff that you buy at your LFS, but I try to avoid putting "treatments" in my tank, so I don't want to recommend it. Wiping it away, do better water changes(bigger and/or more frequent), and don't let food stay in the tank for too long.