Nudi?

What is this little yellow guy from Secret Bay, Bali? He was about 3mm.

https://flic.kr/p/ZUTkTu

Looks like a gastropod mollusc - not a nudibranch or other sea slug.

Definitely not berthelina which is like this :

Berthellina nudibranch by Ludovic, on Flickr

To me it's a sea snail, not a nudibranch nor headshield slug.

OK, I agree, not a nudi or a headshield slug. Sap sucker? I've been through the REEF Nudi and Slug book, and REEF Creature, and can't find anything like it. Maybe a juvenile XXX? What is XXX?

Here is the pix, sorry for posting the link before.