Yellow/mustard algae is more resistant to chlorine so needs higher shock levels to completely kill it off (roughly an FC that is 60% of the CYA level instead of the more usual 40% of the CYA level, but this is unconventional advice on this forum). It also likes shade so will hang out in light niches, under removable ladders, etc. which all need to be checked. Also, one must put in all equipment used to clean the pool (i.e. poles, nets, etc.) into the pool while shocking and should wash swimsuits in soapy water to prevent re-introduction.