I have GSP and a milli on my back glass,
(old pic, that's the same milli on the right)
The only problem with the GSP is occasionally it will start to peel off the glass and then snails just plow under it making the problem worse so I end up having to cut half of it off, right now I have a bunch on big pieces with nothing to do with. It was originally attached with magnets. Maybe my problem is no coralline on glass.
The sunset milli is on a <a href="http://www.bulkreefsupply.com/saltwater-aquarium-fragging-supplies/frag-plugs-disks/oceans-wonders-mag-disks-4-pack.html">Mag Disk pack</a> I was given and I gotta say they work pretty good if you don't want to glue down a frag on glass, pricey though.