I can't immediately rule it out, but . . .

1. Gel coat oxidation is common and well-known;
2. Calcium phosphate pool scale is rare, with only one confirmed case on Google.
3. Phosphate remover companies have latched onto possible phosphate scale as a NEW reason to sell removers.
4. Insoluble phosphate chemistry is nastily complex, with limited empirical data on when scale forms in boilers

I dug around a bit, and found some data, but it will take me quite a bit of time to wade through what I've found. I'm not willing to do so, unless you are first willing to do your part:

1. Take a series of photos of the areas with discoloration, gradually zooming in on a fixed area, till you have reached the maximum macro capability of your camera. Take the pictures in the early AM or late PM so there the texture of any scale will be revealed by shadowing. Email them to me (poolforum@gmail.com) FULL size.

2. Purchase a Taylor Technologies Inc K-1106 Phospate Test Kit
. Test BOTH your fill water AND your pool water. If levels are high, you'll need to dilute in order to get meaningful results. Use distilled water from Walmart: www.walmart.com/ip/10315382

3. List ALL the chemicals you used this season and last, with special attention to stain / scale remover products and any products made by United Chemical. Also if you've used any pH control products that claimed to be 'buffered' or especially long lasting, it's critical that you ID and list those. In the past, some various phosphoric acid compositions have been sold in that manner. As accurately as you can, list quantities.

Best wishes,