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As above says...........
Wipe it off with a brill pad (new one) do it gently..
Is your tank always in direct sunlight?
How long do you have the tank lights on for a day?
it can be due to new tank set ups (usually last 2month IME) or not a long enough photoperiod, have the lights on 8-12 hours a day.
Get some flourish excell. If you get snails check that your water isn't soft, for they don't live long in soft water.