Phyllidia alia Yonow, 1984 Phyllidia coelestis is a species of sea slug, a nudibranch, a shell-less marine gastropod mollusc in the family Phyllidiidae.

The mantle has a grey-blue background color; the margin is covered with many small tubercles.

Bigger tubercles that are yellow capped, run along the median line.

Two of them are on each side of the mantle, and one on a median line ending on the anterior part as a "Y".

This particular line shape constitutes a distinctive element to identify this species.