The exposure to air is a myth.
This animal needs A LOT of LR and DOES NOT eat "algae" in the sense we think of it. IMO, you made the right decision. The odds of long term success in your tank, beyond 18 months, was slim, IMO. Other Linckia may be more reasonable to try, but NONE eat "algae" and all need a lot of high quality live rock on which to graze. Surface area is key, not density.
Simply put, if they ate "algae" they could go in small young tanks, and frankly, none would die of starvation as algae is pretty ubiquitous in our tanks. Yet they do, and they don't do well in small systems.