It looks like a bloom that isn't spreading through the tank. is it below the heater, and the filter intake?
I've wondered about thermoclines forming in aquarium, and this sort of says it's possible.
A thermocline is where water at 2 different temperatures stays seperate and doesn't mix, I have swam through them, warm surface water, a cold layer, then warm water below that, and they are also known to trap sound and mess up sonar systems.
And if it is a bloom it will deoxygenate the water.
How long has the tank been running?
Try moving the filter intake and airstone into the layer. Is the airstine running? And is your filter outlet disturbing the water surface?