I'm pretty sure this is Favorinus sp.
There are several places that describe it as Favorinus sp. 2
(even though this page says sp.1, he identifies it earlier as sp.2). The "Nudibranch & Sea Slug ID Indo Pacific" book describes sp.2 as -
15mm, Orange Red with orange lines on the oral tentacles; cerrata brown with conspicuous lamellae and black apices; cerata orange with white subapical rings and purple tips....
Keep in mind that the species numbers for undescribed species have no scientific significance. XXX sp. 2 simply means that for the given resource (book, website, etc.), the author had one undescribed XXX listed previously. The numbers simply differentiate the given species from other undescribed species of the same genus.
For example, my Hypselodoris sp. 2 found here is the same as what Erwin Kohler calls Hypselodoris sp. 9 here and what is called Hypselodoris sp. 23 in Nudibranch & Sea Slug Identification -- Indo-Pacific. They're all the same species and can simply be referred to as Hypselodoris sp. when identifying a new photo.
As for the photo above, it is indeed Favorinus sp.
- cjames likes this