That's mold. It isn't a problem unless your fish have open wounds, in which case I'd remove the fish to a hospital tank to heal.
Shrimp love this stuff - a few RCS will clean it right up, if your betta doesn't eat them. Providing a mossy hideaway helps, there, and the shrimp chow on the mold much faster than snails (unless you have very large snails).
If mine gets really thick, I suck it away with the gravel vac (I don't have this issue in shrimp-populated tanks) and it eventually goes away entirely.
Diatoms (the brown stuff) can be a pain. I get it a lot in the tanks where the lights are close to the water and I can't adjust the height, but the primary issue is high levels of silicates in the water, which may or may not be helped by upping water changes a bit for a while - and I get this in the tank with gravel, not sand, so that seems not to be a major cause in my own case.
They'll go away, usually, but two of my heavily planted tanks always have some. I scrub it off at water change time, like algae, and it's not such a bother.