trying to get rid of mustard algae.

Time to do a SLAM, you can eradicate the mustard algae (I did this about a year ago) it takes a slightly higher
shock value.

Read about doing a slam here:

Pool School - SLAM - Shock Level And Maintain

But first do you have one of the recommended test kits? TF-100 or Taylor K2006C If you order the TF-100 get the XL option you will need for your mustard slam.
if so please post test results including CYA (very important to know otherwise you are shooting blind)