After re-reading your posts and seeing you tried Kim's paper towel trick, you might consider some of the following:
- Yes, you can have algae even if the water appears to be clear. That's how it usually happens to me. It usually happens to me in areas that don't get good water circulation. For me that's near steps or at the crease/bottom of the pool where the sides meet the bottom. This is especially common (for me anyways) when the pool is not used much because of low activity. Something you might look at is the water circulation in that area. Toss a few ping pong balls on the water to evaluate your water movement. If it's poor, see if you can adjust your return jet eyeballs to help, or just know that the area in question requires a quick brushing each day or two.
- Your CYA is actually low for a SWG pool. TFP recommends a minimum CYA of 70 to help the FC work more efficiently. You wouldn't think that a lower CYA should be a big deal in cooler temps, but perhaps. But don't increase the CYA to 70 yet until you have eliminated the potential algae. The SLAM FC of 20 for a pool with a CYA at 50 is better than increasing the FC with a higher CYA. Be sure to pass the SLAM first, then increase CYA.
- Since you have a solar blanket, you might want to inspect the underside as well if you've used it. Algae can grow just about anywhere. I found some algae a couple days under my poolside skimmer lid.
In the end, even in the best maintained pool, algae sometimes finds its way into our pool for no apparent reason. Even though you showed no FC loss or elevated CC, visually seeing algae confirms the need to SLAM. Make sure the pool is not covered during the SLAM. Let the water breathe. At least this time of year the SLAM should be more efficient as opposed to the heat of summer. Along with the cover/blanket, make sure to check any other potential hiding spots. If you are still SLAMming and weren't aware, you can use regular bleach (no scents or splashless) to effectively conduct your SLAM and save some service on the SWG. Try those tips and let us know if you see any changes over the next day or two. We'll watch for any updates and try to help as needed.