There are plenty of nudibranchs without cerata, but I can't see enough detail to be sure whether it's a flatworm or nudibranch. If it's a flatworm, it's one of the species that mimic nudibranchs (there are a bunch that do). It does look like it has horns or rhinophores, but I can't tell from the pics whether it has an external gill on the dorsum.
Don