Spotted Dick - or a crustaceous lichen

Took the boat gently round the docks today looking at the walls and noticing how lichens grow on the south facing walls and mosses on the north facing ones. 
A tight little description : A lichen is a composite organism that emerges from algae or cyanobacteria (or both) living among filaments of a fungus in a mutually beneficial (symbiotic) relationship. The whole combined life form has properties that are very different from properties of its component organisms.  Rather reminds me of bits of my geometry lessons.

These ones may only grow about 1mm a year - so could be 70 years old but iof course if they've grown more than that in a year they may be younger... Oh! the complications.