I have green slimey stuff on the top of mine, so not that specifically. But it is very managable. When you prune it back just pull the top half. I use it as an indicator it is time to pull some out because it only accumulates when the refugium chamber is getting overly full. Without delving into proper identification (it does look like it could be cyano, but i am not an identification expert by any means), I imagine as long as there isnt something driving the growth of that slime such that it is outcompeting you chaeto that your experience should be similar to mine. Do you have any of that gunk growing in your display?