Flabellina rubrolineata

Yes, for 6 months every year, from April to October I'm in Chalkidiki in north of Greece.

Are you based in Greece for diving?
I visited Greece once but didn't get to dive then, would love to go back and do a bit of diving.

Thanks AlbertKang. I'm diving in Med for work for last 10 years and I never seen this one before. Could not find a ID, but I was thinking it has to be a invasiv specie.

Nice find, Marek.
This species is not natural in Mediterranean Sea but a lessepsian migration species, meaning it originates from Red Sea and migrated to Mediterranean Sea through the Suez Canal.

Wow, great shot... type of Nudibranch??

wow. that's beautiful. and extremely clear and good photo.