That looks like staghorn algae which is a greyish white color. It is usually caused by higher light left on too long and low CO2 levels. Reduce your photoperiod to 6 hours for now and spot treat the algae with either Excel or H2O2 (hydrogen peroxide 3%). Draw up 2-3ml of either Excel or Peroxide to every 10 gallons of tank water, then slowly squirt the algae with the Excel or H2O2. If using Peroxide turn your filters and lights off for 15-20 minutes. If there is alot you may have to spot treat an area each day. Also are your running CO2 or using liquid carbon?