Usually, I get a pinkish color on BBA after I dose with Excel (well, industrial glut diluted to Excel strength). If I spot dose, it goes pink/purple more quickly. If i water column dose, it takes 24-48 hours. In any case, that stuff kinda looks like BBA that has been dosed and is dying off. I may be totally off, but that's what it immediately reminded me of, especially since you mentioned Excel dosing.
If some of it *is* algae after being killed off, it will disappear over the next several days. But I can't speak toward it coming back.