Those are barnacles. They're crustaceans which, after a free-swimming larval stage, attach themselves to a solid surface (sometimes a living being like a whale or a turtle) and become sessile, feeding on plankton with long, feather-like arms. Completely harmless, just kind of gross-looking.
They have the longest penises of all living beings since they only reach sexual maturity after they become fixed.
Darwin once spent four years classifying all known barnacles. He spent hours every day studying them. His kids used to ask other children "So where does your dad study barnacles?'. He once said "I don't think anyone has ever hated barnacles as much as I do."
He did manage to reclassify them pretty neatly, discovering about half of all species were simply the opposite sex of the other half.