That appears to be bryopsis. Nothing that will fit in your tank will eat it. The only apparent solution is to raise your magnesium level above normal SW levels:
Link.
You need to purchase a few other inverts for your "CUC", namely some cerith and astrea snails. Both scavenge for micro algae, detritus, and other leftovers, on the rocks and glass. I would not recommend any hermits
at all, since they are more likely to eat your snails than anything else.
To be honest, you will probably not like what else you find coming out of that rock. You already have quite a bit of nuisance algae, both micro and macro, and nothing to clean it up. You will go through algae cycles: diatoms, cyano, maybe even a hair algae outbreak, and the tank will only look nice if you are diligent in your husbandry practices. At some point you'll probably see bubble algae too, which is absolutely impossible to completely eradicate. Removing algae daily will definitely get tiresome, trust me!
If you want the ADA-look with a saltwater aquarium, you will likely have to start over with a better initial investment, much as you have already alluded to. You can certainly get rimless aquariums from other sources, and there are ways to hide equipment other than a sump. (A closed-loop manifold containing a heater would be one such idea. Water movement and heating capacity with only a few holes drilled; or no holes drilled if you go over-the-back with some lily pipes

)
Cheers,
Marty