Are you sure you have a mustard algae problem ? My mustard algae would come back the next day or two but never a couple hours later, that sounds like perhaps you have DE or dead algae in the pool. My mustard algae returned on the sides of the pool in certain spots and on a grotto shelf. I finally got rid of it by getting FC up to 25 ppm and using polyquat 60.

What kind of filter do you have ?