The Rockgrub is tiny in size, its translucent body glows a bright bioluminescent green color. It lights up a small spherical area around itself. It has a slim, round, segmented body with two swimming fins on either side, and a larger flipper serving as the tail.

The Rockgrub has five appendages on each side of its body, and a circular maw containing many small teeth.

The Rockgrub's small size would allow it to consume small bits of coral or vegetation clinging to the cliff faces near it