Your experience lends further support to the theory that SWG pools can have a lower FC target because free-floating green algae gets super-chlorinated in the SWG cell. Yellow/mustard algae tends to settle and not free-float so won't get circulated as readily. So that would make the manual and SWG chlorine levels similar for that algae, namely an FC that is 15% of the CYA level.

For most SWG pools it is better to use a higher CYA level of 60-80 ppm since that will have the chlorine last longer (even at higher FC levels) and let you turn down your SWG output (since less absolute FC loss per day) which helps reduce the rate of pH rise. However, in your situation, that would require (in theory) an FC of 9-12 ppm to keep the mustard/yellow algae from growing.

This is one of those rare times where one of two things can be done. One is to super-shock the pool at 60% of the CYA level so in your case probably 25 ppm FC to be conservative and hold it there for several days and put everything and anything into the pool that might be harboring the algae, such as poles, etc. Even remove the lights from their niche to expose chlorine behind them. If you completely kill off the mustard/yellow algae, then it may not come back even at more normal FC levels.

The other option is to have your phosphate level checked and use a phosphate remover if its high (or even if its not, in this case). This is rarely suggested but it is another option to allow you to get to a lower sustained FC level. It's up to you what to do.

Richard