It seems to be dinoflagettes.
They are photosynthetic so they do thrive under light.
There may be no detectable NO2 or NO3 but you could have a high DOM or phosphate level. Did you test for those?
It is pointless to just change water... you have to use a toothbrush to scrape them off and syphon as much as possible.
If not, make sure they are suspended in the water column and use filter wool to strain the water. Once the water is clearer, throw away the filter wool to avoid them reproducing or reintroducing them back into the system.
Those fur like stuff that is not moving is a strain of hair algae.
If it moves quickly like little fleas, then its pods, which is good.