Strange algae in my fresh water tank.?

i have a 55 gallon freshwater tank full peaceful fish as well a several fat catfish. it's been established for about 8 months. the water is very clear, we have about 12 plants and 2 moss . but the past month or two i have noticed strange algae, or at least i think it's algae. it gets EVERYWHERE. its fluffy and grayish-brown and attached to nothing. it sits on the floor or in between plant stems but if you swish the water around it is easily picks up and swirls around the tank. its gets really big and starts to take over the tank floor, it is very ugly and we've tried removing it manually [literally scrubbing the driftwood and leaves and shaking up the rocks on the floor], giving less light and lately switching out more water. we're out of ideas.. please help!