I would think that's borderline cheating, yes. It's shady. You're hiding the fact that you're talking to another girl. What is there to hide, unless you're breaking "the rules" and doing things you shouldn't be doing behind your partner's back?
Its a very old technique... and yes, it is cheating. He didn't even make it known that the contact was female. Thats deception.. aka.. cheating. Cheating is not an instantaneous thing, it builds.. you just caught one on the building process. Thing is, will you stop it... or wait for the act.
First of all, there is nothing wrong with talkign with other girls, and even as your girlfriend you have no right to demand him to tell you everything he does. He has the right to have a life on his own too.
Secondly, why would he do it? Could be his girl is so yealous it'd be hell on earth even if he's marginally talking to other girls? Some say the fact he's hiding it means he's doing shady stuff. What if he's being pressed into hiding it, because he feels hiding is better than having a girl go überbitch mode on him?
I can't say i know much about snapchat, but who defines genders? And secondly, HOW do you know this? Snooped on his phone? What reason did you have to snoop on his phone?
For those interested in having my heads ofr this; yes, i am playing the devils advocate, because i want all facts on the table.