I also say it's a slime mold. Likely, the bacterial film is normally transparent and dispersed throughout your decor and substrate. When it is shocked, it clumps together until it adjusts to the change. Sort of like when you poke a sea anemone and it retracts for awhile.
I guess this simply because it looks a heck of a lot like the blue-green algae I get once in awhile (also a bacterial slime). The next time it's "gone", try shining a black light on your tank. You might see layers of green ribbon hanging invisibly in the current. Also, watch all the leaves and mobile objects in the tank and look for mysteriously strong movement that couldn't be caused by the available current but would have to come from a cumulative effort of, say, a long trail of slime being pulled on.