The colors are like flame algae but I've never seen that pattern before. Very cool! Let us know if you find out what it is. I wonder if tangs and angels would eat it? I'd like to get some if they don't otherwise it will sit in the sump and I wouldn't see it...
Red Sea Lettuce has smooth edges and the picture of the OP's is jagged and has a pattern in it. Are there different sub-species of Halymenia porphyraeformis?