Gray mung of some sort

[nightmarepatrol]

We just opened up the pool Saturday after it being covered by the solar blanket for about 3 weeks) and found stuff on the pool walls. I could not brush it off even with the metal brush. The end solution was to get into the pool with the pressure washer and blast what I could off the walls so this is not staining. Even then it was difficult and there is great deal remaining especially at the lower levels in the deep end.

The pool was spotless and in balance prior to the blanket going on about 3 weeks ago.
When the blanket came off Saturday the water was tested and it came up with the norms. (Sorry I can't provide exact numbers)
There has been no water added to the pool recently (aside from rain water)
There was zero green or black algae in the pool.
I shocked the pool with liquid chlorine and stop yellow yesterday after my swim with the pressure washer
The gray whatever-it-is still cannot be brushed off.
The pool temperature is roughly 80F

I am located in the gulf coast of Florida 4 miles from the Gulf.

I have a 10K pool with one main, one skimmer and three returns. the surface is diamond-brite and is approximately 5 years old.

Any ideas what I'm dealing with and how to get rid of it?

Thanks

 

[JasonLion]

Welcome to TFP!

Could you post a picture of this gray stuff?

 

[nightmarepatrol]

This is not a terribly good picture. since it was taken in the morning light. You can see the pressure washer marks where the stuff has blown off. The water is still cloudy from the amount od detritus stirred up yesterday. I also failed to mention I am using a hayward sand filter.

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