At 100 ppm CYA, typical FC loss is around 15%, but you are at 20 ppm so you're doing even better than the predicted 3 ppm loss. If you didn't have yellow/mustard algae, your minimum FC would be more like 7.5 ppm in which case you'd only be losing around 1.2 ppm FC per day which is lower than the typical pool (2 ppm is fairly typical; 3 ppm is not uncommon).

You should check behind light niches and under removable ladders. Yellow/mustard algae prefers shades and often hangs out in areas with poor water circulation. You need to completely kill it or else it will come back when you let the FC level drop below roughly 15% of the CYA level.