What you have is exactly what I was fighting a few weeks ago. Mine got to the point of about 80% surface coverage. Never did have a smell, so I ruled out BGA from that viewpoint. Never did get an answer from anyone as to a possible ID, either. I assumed it was a reaction from the initial addition of pond dye...killing off a bloom.
Its nasty stuff regardless. Bottom aeration pushes it out away from the boil, but it doesnt dissipate. I assume it rises as its dying and may eventually go away on it's own, but the only way I could get rid of it was copper sulfate treatments and then the fountain to keep it down. My last visibility reading was 36" and increasing.
I've noticed that late of the evening, the grayish oil slick looks like it wants to come back, but when the fountain comes on, it knocks it back down.
If you get a solid answer as to what this stuff is, I'm thinking there's more than just myself that would like to know.
.10 surface acre pond, 10.5 foot deep. SW LA. The epitome of a mutt pond. BG, LMB, GSF, RES, BH, Warmouth, Longear Sunfish, Gambusia,Mud Minnows, Crappie, and now shiners!!...I subscribe!!