Magnesium doesn't kill it...tech M used to but it wasn't the magnesium...it was a contaminate in it.
UV doesn't effect what's growing on your rocks. It only effects what flows through it.
When mine came back I removed most corals and dipped my rocks that had it in tank water + some peroxide to make it bubble for 10 min.
I would try reeflux first only because most people are just not brave enough to do what I did.
Keep an eye on the fuge...if it starts to die then toss the macro and get new macro after the treatment. Keep an eye on your fish...the reason I didn't try a 2nd treatment of reeflux is one person reported losing all their fish but that's not common. Probably effected oxygen.