"Comment from Nathalie Yonow: This appears to be a juvenile Phyllidia varicosa, very similar to one I have from the Red Sea. It doesn't have the bar between the rhinophores of Phyllidia alyta, with which it might be confused. Unless there is a photo of the ventral side, we can never be certain!"
Comment from Richard Willan
Comment from Nathalie Yonow: This appears to be a juvenile Phyllidia varicosa, very similar to one I have from the Red Sea. It doesn't have the bar between the rhinophores of Phyllidia alyta, with which it might be confused. Unless there is a photo of the ventral side, we can never be certain!