Some thoughts.
Green hair algae that is infested with something, dinoflagellates maybe, would look like the hairy stuff. Gently pull out some of those filaments growing in the sand and place them on a microscope slide in a drop of tank water. Place a cover slip on the drop of water but do not smash it down. If it is GHA, you will see green branching filaments. Stuck to those filaments could be all sorts of small organisms whose diameter will be smaller than the width of the GHA filament. There might also be nondescript clumps of “stuff”. There could also be cyanobacteria filaments entangled with the GHA filaments.
The beautiful green covering on the rocks and glass could be another type of algae. Getting a sample of these to view under the microscope could be tedious, worth at least a fifteen minute attempt. The ones on the rock could be different than the ones on the glass. Either of these algae might be a swimmer that settled, but you won’t know this until you view it.
Post what you find.