i cant really tell you what it is, but since it is so raised i would just manually take it off for now, just rip i off or pull it off when ou can, that shouuld keep it in check till you find out what it is
If the rock is small and has no corals on it I would just sacrifice the rock to kill the algae by taking it out of the tank and allowing the rock to dry out/or manually scrub it off with an abrasive brush.
You could also "cook" the rock, meaning put it in salt water in the dark with a powerhead present for more than a month (until the algae completely dies from lack of light).
Well I picked up 5 Mexican Turbos this past Monday. They went nuts and completely cleaned off my Seios, Overflow, and back wall... in all of 4 days. So maybe they will turn their attention to this stuff....
As for the Toxins... yeah well I stopped using my sump as a urinal... maybe things will get better soon. The extra foaming action it created was great for the skimmer.