Placida cremoniana (Trinchese, 1892)

Current taxonomy; World Register of Marine Species
www.marinespecies.org/aphia.php?p=taxdetails&id=141564
Probably the first United Kingdom record of this distinctive Mediterranean species. Apparently rare in the Algarve, Portugal (Calado and Silva, 2012). Not recorded in Galicia, Spain (Trigo et al. 2018) and no record nearer to UK known to this author (please inform if you have contrary evidence).
This seaslug is not a nudibranch. It is a sacoglossan, feeding on green algae as visible in this image.
There is no other similar species recorded in European waters, but three very similar ones live in the Pacific, and additional species may occur in the Indian Ocean, tropical Australia, and Caribbean.
Sublittoral, 8 metres. Falmouth, Cornwall, England. 13th November 2020. © & Leg. Shannon Moran, det. James Lynott, conf. Dr Cynthia D. Trowbridge.

References
Calado, G. and Silva, J.P. (2012). Sea slugs of the Algarve. EdiçõesSubNauta

McCarthy, J.B., Krug, P.J. & Valdés. 2019. Á. Integrative systematics of Placida cremoniana (Trinchese, 1892) (Gastropoda, Heterobranchia, Sacoglossa) reveals multiple pseudocryptic species. Mar Biodiv 49, 357–371.
link.springer.com/article/10.1007/s12526-017-0812-2

Trigo, J.E.; Diaz Agras, G.J.; Garcia Alvarez, O.L.; Guerra, A.; Moreira, J.; Pérez, J.; Rolán, E.; Troncoso, J.S,; Urgorri, V.. 2018. Guia de los Moluscos Marinos de Galicia. Servicio de Publicacións da Universidade de Vigo.