Click the thumbnail photos to enhance!
Pretty similar is Phyllidia varicosa which has tubercles forming many longitudinal ridges, yellow-capped tubercles at the mantle margin and a dark longitudinal stripe on the blue-grey foot sole.
More information on Phyllidia coelestis are on Bill Rudman's Sea-Slug Forum!
How to cite:
Köhler, E. (2017), published 1 December 2017, Phyllidia coelestis Bergh, 1905
available from https://www.Philippine-Sea-Slugs.com/Nudibranchia/Euctenidiacea/Phyllidia_coelestis.htm