But if it is a new tank, and if you can stand it (big if there I know), ignore it for a few weeks and see if recedes as the tank matures. Many "young tanks" develop diatom films all over the place, and in the majority, but not all, much of the diatom/brown algae will fade as the tank matures. If your water supply has high silicate levels, you may be one of the folks where the stuff hangs on - your utility will know whether your silicates levels are high or not, call and ask.