Is it really an algae?
Algae photosynthesizes, so it will always have some color. Often green, but not always.
Fungi do not photosynthesize, so will usually be white or clear. Dirt can get stuck in fungi, so it can appear grey.
If your tank is new, brown algae is likely to be your first encounter. Water changes and scraping your glass, rinsing your decor with tank water, feeding fish less, will all help. Either way, identifying what type it is would be your best bet. Each has a different cause and effect.