It's hard to say what that one is doing. Did you buy that one or was it a hitchhiker? It seems like we only know what a few species of nudibranchs eat including sea hares and then the pest ones that eat montipora or zoanthids.
I guess I'd watch the zoas to see how they act. If you can see him munching on them, I guess you figured out what it eats. If he's just moving around, the zoas will be fine.