Looks like it might be some form of turtle grass....

Ben brought over some of my macro algae to put in your sump right? That stuff should do a good job outcompeting it for nutrients and eventually it should recess. Just keep getting it pulled back as much as you can until then.

If you had a strong population of herbivorous isopods & amphipods they would probably ravage that stuff. What are the chances I can have some next time you do some pruning David? :grin: