It can only kill what goes through it. That's why people only use it for green water. I turn mine on when I clean my glass too, as that will go through the UV as well. Again, you can't easily get rid of much algae with it only because it can't go through it. I am sure it will kill any algae if you had a way to force it in, it just doesn't make sense to do so.
Yes, as was said, it will kill any algae free-floating in the water column - and that includes GDA (green dust algae), but you need to remove it from the glass by brushing and get it into the water column first.