Phylum: Ochrophyta   Family: Ralfsiaceae

Ralfsia verrucosa (Areschoug) Areschoug, 1845

Limpet Paint

A very apt common name as it's often found encrusting on old limpets on the lower shore. It also forms orbital shapes in rock pools where the colour can be more ochre

References: 
Fish & Fish (2011) - A Student's Guide to the Seashore;
Bunker, et al - Seaweeds of Britain and Ireland