With your CYA of 70, your FC is way too low, at this level you should keep it maintained between 5-10ppm FC. Your going to have to boost your FC level up to at least 20 ppm to get rid of the mustard and keep it at 20. Along with daily brushings and keeping your filter running 24/7. Otherwise, the mustard is going to keep coming back. I fought with my pool for 2 months with mustard algae, before I got serious about getting rid of it for good.