sounds like a mold issue. syphon it out next time you do a water change and vacuum the bottom of the tank thoroughly. mold flourishes on dead bits of food in and on surfaces in your tank. remove the food source and the mold will be gone in a coule of weeks.
i will due in about two days for a water change it doesnt seam to bother fish, i have a parrot fish that is always begging for food and sometimes im a bit weak. thank you