Nah, looking again its far to big and dark to be riccia. I have some in another tank that has no contact with this tank and its totally different. Plus it doesn't pearl, which riccia tend to do a lot under high light and co2.
I've updated the topic title now, feel free to move the topic too.
My money is still on some form of pellia.. It came out in one go with no breakage too, so I wouldn't say it was that fragile either... Its in my nano now, so I'll keep an eye on it and see how it develops.
As Graeme said, I've seen it and had it, real sure it's a type of Riccia. Given it's dark colour I thought it was just dead Riccia, until it kept growing! TGM had it in their old Iwagumi and i believe it's almost impossible to get rid of in that kind of situation.