Now is when all your test numbers come into play which is why everyone keeps asking for them. If it is indeed algae, especially mustard algae, we will definitely need to know what your FC, CC, PH, TA, CH and CYA levels are before proceeding. Maintaining the proper chemestry during the high chlorine levels that will be required to kill it is important so we will need them for this purpose and to know exactly how high the chlorine will need to be.
Please report those to us and then we can formulate a plan to help you get it cleared up.