A 3-4 inch sandbed can also be a source of high unprocessed nutrients. You may want to really stir the sandbed up and then replace the filter pads. I would do this a couple times.

A sandbed should have a depth of over 4 inches or 1 inch or less, with preferably Sugar fine sand. Also you do not have enough current to maintain a deep sand bed (DSB).

In your situation, one solution would be to remove much of the sand, and conduct numerous water changes. Removing the sand will make a mess.

This would be one way to help with long term excessive nutrients. However it will not kill the algea immediatly these things take time.

I would look at increasing the current and your cleanup crew.