Looks like a fungus!!

[merlin39]

Sits around 7cm across and growing...

Any thoughts??

[szymek]

Looks like "Padina - funnelweed" starting to develop.

Can be nuisance if not controlled. Otherwise nothing to worry about. Just need some herbivores to keep it in check or manually remove it. Usually when you start picking at it, it will die off.

Well that is if my ID is correct.

[BigAl]

:iagree: Also known as Scroll Algae. Just prune it back now and then and it will play nice. If you were closer I would happily come take it off your hands. :-)