... Does anyone have any other suggestions?
(1) Manually remove as much algae as possible and perform 50% water changes 2x a week until the algae begins to recede.
(2) Increase plant density - an easy way to do this is to add some floating plants.
(3) Dose 3x the regular dose of Excel for your tank size and don't forget to double dose with every water change. If you are injecting co2, make sure that you have stable C02 levels(use drop checker to test).
(4) Reduce or split your lighting. If you run your lights for 10 hours go down to 8, or you can split this using a timer. 5 hours on, 2 hours off, and 5 hours on, or 4 hours on, 2 hours off, and 4 hours on. IME, plants continue to do well when you do this but algae does not.
(5) I know that you don't want to add fish, but if your water parameters are normal (no ammonia spikes), consider adding Amano Shrimp. These guys just love most kinds of algae and will greatly help with your clean up efforts.
Above all be patient
. With some of the tanks that I set up, things did not come around and I thought of writing off the tanks and tearing them apart. Much to my surprise, after several months, the tanks appeared to stabilize on their own with no major changes on my part, other than the addition of some Amano Shrimp and oto(s). The algae began to disappear after many months when it looked as though it never would.