Purple Agae -What is it ?

[DiverDude]

So I seem to be having a bout of this deep purple algae on the glass of my tank (sides and back panes only). When I scrape it off, it comes off in sheets (and the sheets float around and stay intact).

My Alk and Mg are slightly off (3 meq/l and 1075) but other params are where they should be. BUlbs are coming up on a year next month so that could be part of it...