Probably a bryozoan. Some of these critters have been spread about and can be a real pest.
Not the right colour and not the one you have but they can look like this or frilly fan like growths.
This is written up in some introduced pest info about one species "introduced populations have been recorded on the West coast of the United States, Hawaii, Australia, New Zealand, Europe, and South Africa. This species is known from rocks, oyster shells, pilings, floats, oil platforms, ships' hulls, and fouling plates. It is tolerant of copper and mercury antifouling paints and has outcompeted congeneric species in some areas of its introduced range".
I get heaps where I am and I'm not too happy with the constant diving needed to keep them off.
