When my latest liner was installed a few years ago. they used a pad under the liner and taped the seams of the pad together with duct tape. After the liner was installed, I noticed that dirt wanted to collect in those lines. I can vacuum and the next evening, the dirt is back in the lines. When we swim, it gets stirred up into suspension and you can't see it, but when the water settles down, it again forms in those lines. It is just dust and dirt and not algae. Maybe that is what is going on with your pool.

Is the floor of your pool slimy feeling at all?

Do this so we can rule out algae:

Tomorrow evening when the sun is off the pool, take your chlorine reading up to about 10ppm. An hour or two after you add the bleach, test to verify what your chlorine reading is. Then, the next morning within 2 hours of sunup, test the cl again. If you don't lose more than 1ppm of chlorine overnight, then it is not algae. Report back what you find out.

By the way, you need to lower your pH a little. If it is higher than 7.8, it is too high. Add about a pint of muriatic acid slowly in front of a return jet. Wear gloves, glasses, stay upwind of fumes, pour close to the water surface to avoid splashing. A few hours later, retest pH and redose with more acid as needed until you get the pH down some.
Do not test the pH when your chlorine level is high, though, or you'll get a false high pH reading.