Just to confuse the issue - is this just at the glass that you see this? And does it appear fairly even over extended distances along the glass, paralleling the sand surface?
If so, a significant part of this may well be only on those sand grains or the glass itself exposed to internally relected light. Light from the tanks lighting entering the glass at certain angles passes through the interior glass surface, but is reflected back from the exterior and comes back out within the tank just below the substrate surface - the glass itself acts a light pipe for a short distance. In one small area, pull back or siphon up the sand, if the algae is still there in the glass with a bit in on the sand removed, it is just internal reflection promoting growth and it may be removed at will. If the patch extends into the tank away from the glass a completely different procees is occuring.
