Melobesia mediocris is a coralline crust associated with surfgrass and eelgrass. If I had to pick a common name, I’d probably go with surfgrass coralline. Seagrass crust is a reasonable option. Young discs (center in the image below) are round and remarkable for their beauty. On Phyllospadix (the green blades below), Melobesia‘s growth is constrained by its host’s narrow blades, so larger patches develop a more or less rectangular shape. Mature crusts are thick and pitted.
On the outer coast, look for M. mediocris on low intertidal surfgrass. If you are a beachcomber, check for it on drifted surfgrass and eelgrass, Zostera.
Even at a distance the pink tinge of Melobesia on the Phyllospadix‘s green blades is noticeable.
Druehl, L. D. and B. E. Clarkston. 2016. Pacific Seaweeds: A Guide To Common Seaweeds of the Pacific Coast. 2nd ed. Harbour Publishing Co.
Lamb, A. and B. P. Hanby. 2005. Marine Life of the Pacific Northwest. Harbour Publishing.
Mondragon, J., and J. Mondragon. 2010. Seaweeds of the Pacific Coast. Shoreline Press.