Help me determine what this is - White mold? Algae? Calcium? Something else?

[jyo1]

I've recently taken over management of my new pool and after a drain and refill, am focusing on getting the water chemistry in shape (you'll note in my results below the CYA and salt levels are low for this pool, which has a SWG. Note that the SWG is not turned on yet - I've been adding liquid chlorine while I raise slowly those levels.)

Last week, I noticed a white substance which in the corner of our spa. as soon as I disturb the water, much of it very easily breaks up.

From another thread, I thought maybe maybe this was calcium, but adding a drop of vinegar didn't cause any fizzing (see video).

Anyone have any idea what this is? This is in the corner which does not get a ton of circulation. The pool is covered by a tight-fighting opaque automatic pool cover 23+ hours a day - I thought maybe we were just creating a very stagnant corner of the pool but, but I haven't seen really anything that looks like this before.

I've attached a few photos, as well as linked video of me scraping this stuff, as well as the vinegar test.

Scraping:

broken-up-vid.mov

Shared with Dropbox

www.dropbox.com

Vingar test:

vinegar_test.mov

Shared with Dropbox

www.dropbox.com

Any help you can provide would be greatly appreciated!

==========================================

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Build Type: Plaster
Volume: 5900 gallons
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Latest Test Result Summary:
FC: 5.2 (36 minutes ago)
CC: 0.0 (36 minutes ago)
pH: 7.8 (36 minutes ago)
TA: 110 (36 minutes ago)
CH: 230 (36 minutes ago)
CYA: 30 (36 minutes ago)
SALT: 1800 (2 days ago)
==========================================

 

[reggiehammond]

Any chance that's sunscreen or something else from the house?
Does the cover have any evidence of this mystery goo?

 

[jyo1]

I can say it's definitely not sunscreen - spa hasn't been used in the daylight and the cover is usually closed. (Plus, it returned in the exact same location a day or two after removing it initially)

The cover doesn't have any evidence I can see when the cover is closed.