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You must have hard water. It is a build up of the solids that are in the water. You can get it off with vinegar. Be sure to keep you media wet while you clean it off and to throughly rinse the filter off before putting it back on the tank.
I have the same issue with the spray effects on my glass canopy. I just cleaned it off last night using white vinegar, it came right off and the canopy looks good as new