The balance between light, nutrients and waste is off.
I am not an expert, but this is what I would do today
The algae needs to be starved off, I would try the following:
1./Check how much you are feeding, not feeding for a day or 2 this week, not a bad idea.
2./Turn of the light for a few days.
3./ Do a 40- 50 % water change and gravel vacuuming, today and again tomorrow (treated water)
4./ Change 20-25 % water weekly
5./ Temporay use of UV works well but is expensive but then so is diatom filtration
John