@ahiggins , I have these. They are not coraline if they are like mine. Mag-float will not remove them. I have a (DIY) razor blade holder glued to my mag-float and it does (easily).
Is yours a brighter green than the normal glass slime-algae?
If you don't remove it, it will spread slowing outward, but is still fairly easy to remove.
Recently, I have been letting my NO3 creep up to about 7ppm-NO3 and I had a huge PO4 spike (10 PPM). While PO4 is back down to about 0.030 PPM, I notice the back glass is nearly completely coated with this hard to remove bright green algae. This is because I clean the back glass about every 2 weeks. Leaving it on there so long has made it harder to remove with just a razor blade glued to a mag-float. I have to stick my hand in there while holding the razor blade in an aluminum razor blade holder.
BTW, my yellow tank now identifies the razor blade as "Yum-Yum special treat time". Meaning: the yellow tang love to eat this algae when I scrape it off the glass.