Goniobranchus tinctorius (Rüppell & Leuckart, 1830)
Red-netted Goniobranch is called “Sarasa Umiushi”. Umiushi meaning Nudibranch or Sea slug in Japanese. This is a common nudi at Owase. It looks like wearing red lace knitting. It may be named after as that looks.
According to Wikipedia, Sarasa is a dyeing and weaving term that refers to a cotton-colored pattern-dyed product of Indian origin and similar pattern-colored dyed products produced in Asia, Europe, etc. under the influence thereof.