The Mushroom coral reticulidia (Reticulidia fungia) is a species of sea slug, a dorid nudibranch, a shell-less marine gastropod mollusk in the family Phyllidiidae.
Reticulidia fungia grows to a maximum length of 40 mm. It does not grow as large as Reticulidia halgerda. Reticulidia fungia has broad-based ridges, those being fewer in number than those of Reticulidia halgerda. The base of these ridges have a fine, white border. The mantle margin is broad and blue-grey in colour. It has a single, black line around the side of its foot occurring slightly above its gills.
Reticulidia fungia occurs in areas such as: