OK, I've been thinking more about this and I believe you do still have a chlorine demand with this (presumed) algae. It's probably a demand of 1 ppm per 8 hours or maybe a little more. That would "seem" like the chlorine holds overnight, but if you lose half from the sun 15-->7.5 and then another 2 or more from chlorine demand from the algae, then that would get you to 5.

So, I would suggest really hitting this algae with a lot of chlorine. Start with 30 ppm FC in the morning. If you are able to stick around and add more chlorine during mid-day, do that, otherwise add more when you get home at night. Keep this up until 1) there is no more visible red dust and 2) the chlorine holds solidly overnight (virtually no drop at all) and holds more during the day so it drops no more than half.

If that works, then the question becomes how to keep this algae away ongoing. Though you could maintain a minimum of 6 ppm FC at 45 ppm CYA (or 8-9 ppm FC at 60 ppm CYA) which is what some others have found is required to keep away mustard/yellow algae, there is another way that might let you keep lower chlorine levels. See this thread for the use of 30-50 ppm Borates in the pool which act as an algaecide (as well as an additional pH buffer). The problem is that we don't know if this is a good algaecide specifically against mustard/yellow algae though it does inhibit a variety of different algae. Anyway, I just wanted to give you a long-term option that might help since this is a repeated problem for your pool that would require higher chlorine levels to keep away (even at 30 ppm CYA, you might have to keep a minimum of 5 ppm FC to keep away mustard/yellow algae instead of the normal 3.5 ppm FC mid-point recommendation).

Richard