There are 2 things you might want to try:

1. Do the DE test on your filter: dump a quart of DE powder into the skimmer, while the pump is running. If the DE shoots back into the pool via the returns, your filter needs servicing (more sand?) and will NOT remove the algae till that's done. Otherwise, the DE will help capture the algae. This will cause your filter to stop up quickly. Let the pressure rise 5 psi or so over the 'clean pressure', then backwash and add more DE.

2. Once you've verified filter function, and made sure your chlorine is high -- try turning off the pump for 24 hours and testing whether the dead algae will settle to the bottom. If it does, vacuum it to wasted.