Sully is right, that your FC is way, way too low. You can try the 20 ppm FC to see if that kills the mustard/yellow algae, but users on this forum have found that this particular algae (which is chlorine resistent) takes a LOT more chlorine to kill. A couple of users needed to add enough chlorine (chlorinating liquid or bleach) to get to 1.0 ppm disinfecting chlorine (HOCl) concentration and with 70 ppm CYA that would take 47 ppm FC. Regardless of whether you target 20 or 47 ppm FC, you should lower your pH first to make the addition of chlorine more effective. You could first lower your pH to 7.2 (or even 7.0, but not lower) by adding Muriatic Acid. Then add lots and lots of chlorine. Let us know if the 20 ppm FC kills the mustard/yellow algae or not. I'm trying to keep track of the minimum level of chlorine needed to kill and prevent this form of algae.

After you've killed this algae, to prevent it from returning will likely require at least 5 ppm FC as Sully said, though in a non-SWG pool users have reported needing the equivalent of 11 ppm FC (for the 70 ppm CYA in your pool). Again, you'll have to experiment for your pool to see what level of FC will keep the mustard algae away, but it certainly will be far more than the 1 ppm FC that you had before.

Richard