Hey KB? Welcome to the club and the opportunity to walk into a bad arse tank like that! Perams look solid...and I think you are right with that big nem...in the right flow it looks like it could reach that acropora.
It may be wise to move and re-mount the acro a few inches to the left. Secondly it could potentially continue to necrose further pending the severity of the sting. I would monitor closely and if the necrosis continue to spread...I would clip that branch BELOW the run/necrosis some In hopes to prevent it any further. Hope that helps!!