Coralline comes in all different colors. If it won't come off the glass easy then that's what it is. If you've ever had cyano
or known someone with it, you can take an old toothbrush and it will come off fairly easy. I used Purlple Up in my tank and went up this morning and now have some growing on my front glass that my algae magnet will not take off of course...sighs.. Cyano
will usually spread to your sand as well. Just had a buddy get rid of it completely after deciding to use "the vodka method" to help with nitrates and phosphates...BIG MISTAKE!