I have a similar situation in my 14G biocube. I only had 2 red leg cortez hermit crabs which didn't do nearly enough to handle the algae load. Threw in 4 more last Saturday and already seeing a big reduction. One even took initiative to eat through the algae weaved in with my green start polyps and they look much happier now. Also doing better with feedings, going slower and bit more target feed. Was getting lazy on that process and it was helping with algae growth.
IMO, a clean up crew is treating the symptom not the disease. I would guess that you have an imbalance in your nutrients. In other words, you have more nutrients in your water than your fish and invertebrates are able to consume therefore algae makes up the difference. Suggest starting with water tests especially for PO4 and NO3
What CUC do you have right now? What size display tank are you running. Lots of fish eat algae pretty voraciously, but options are limited in tanks <100g.
That looks like straight up GHA. You might have to knock it back a bit with a brush, but my turbo snails devour the stuff so long as it's young and tender.