This hydroid, Hydractinia echinata, is usually found on the shell inhabited by the Hermit Crab, Eupagurus bernhardus. However, it is occasionally seen on whelks and under rock overhangs. It is particularly dense on this crab and can be white with faintly pink polyps or sometimes brown. It is very common all around our shores. There is no recognized common name although I have seen "Snail Fur" used which seems pretty appropriate.
St. Abbs Marine Reserve, Berwickshire. E. Scotland