Are you sure it's not just cyano? It can come in various colors and when on substrate takes on crust/jello like appearance. If it's cyano, water changes + removing by hand (which you already doing) is the best route. If you don't have any clams, you can also add GFO. Running SeaChem PhosGuard mixed in with carbon pretty much eliminated all cyano in my tank. The reason I mention clams is cause suppousedly they can get pinched mantle disease from GFO. Got a pic?