Can you take the rock out?
If so then you can try the peroxide method.
You take the rock out, scrub as much of the algae off with an old toothbrush as you can, then use a syringe to apply peroxide where it used to be to kill anything left on the rock.
Then wait a while for it to stop fizzing, rinse with a tiny bit of tank water then put it back in.
Be careful with the peroxide, especially if you use anything over 3% strength...it will bleach worktops/carpets etc and burn skin if you splash it.
BTW if that's bryopsis rather than hair or turf algae, you might want to just order some reeflux/fluconazole...save some time.