I was having the same issue and i bought a small ball of Chaeto, less than a month in and i had a buckets worth of it grow in my sump, sold most of it.
few weeks later the same result, its been growing pretty well in my sump.
I run a EShine Aquasun 24x3w LED light over the middle compartment of my sump where my Chaeto is 24/7, in there i also have hairline algae growing, to avoid it going into my display tank i have 3 filters going.
Fist layer is a mesh with Polyester Fiberfill, got a huge bag at walmart for like 8 bucks, i then run some soft of plastic strips all tangled together, these break up bubbles and also trap any algae that gets past the polyester, then i have a Foam Aquarium Filter, this is my last defense before the water goes back into my DT. But as a backup plan i have plenty of Algae eating hermits and two tangs.