How much and how often are you feeding? Most tend to overfeed without knowing it. Algae needs 3 things to grow: 1.) nutrients (nitrates or phosphate) usually from overfeeding 2.) light and 3.) water
Scrub off as much as you can with a toothbrush, syphon it out doing a 25% water change. Replace with nitrate/phosphate free water (check water source to be sure). Reduce lighting time to 8 - 9 hours. Most people only feed a small amount every other day. Trochus snails and turbo snails will help as well.