In late summer and early autumn, juvenile Tritonia hombergii appear in large numbers and settle on the soft coral Alcyonium digitatum (Dead Man's Fingers). The soft corals polyps always retract their polyps (as in the picture) when this occurs to try and protect them from being eaten by the nudibranchs. Despite adults being of various colours from white and yellow to brown, I have never seen a juvenile in any other colour but white.
St. Abbs Marine Reserve, Berwickshire. Scotland