Tentacles of a seashore-dwelling rodent. A solution of strained and pickled Murtlap tentacles, yellow in color, soothes and heals cuts and other wounds.
After Harry left his detention with Umbridge with cuts on the back of his hand, Hermione gives him a bowl of Murtlap essence, which helped the pain immensely (OP15). Harry later recommends the same treatment to Lee Jordan, who in turn suggested it to the Weasley twins when they were seeking a solution to the problem of the boils caused by their prototype Fever Fudge (OP18, OP26)
It's funny to think that Hermione, who was always disapproving of Fred & George's inventions, inadvertently helped them finish their Skiving Snackboxes.