This thing alone doesn't mean much. He might be just friendly in general and curious, and noticed you're shy so he enjoyed teasing you, but that could be it, with no bigger intention behind. I don't consider this to be "flirting" already, maybe it's just his way to be nice or funny...
If you give him access to next steps you'll discover if he would push it further or not. Which means: let him add you on social media, or show him your face for a couple of seconds, and see if he tries to know more about you personally or will just keep being a "regular colleague" once his curiosity is satisfied. If you're shy and reserved I'd show your face instead of adding in social media because it gives no bundled access to your personal life and circle of people close to you, past and present, details of any sort.
Also, you're kind of "creating" the circumstance to make him insist: you're contributing to create this game where he is challenged to see your face and you say no, while it's a so extremely basic and normal thing to show your face to anyone else (if there wasn't covid), but you're making it hard like if it's a trophy for him, so probably he is teased by that, too, because it does look like a game honestly. Well, first of all he shouldn't insist, because who is responsibles for "no means no" accidents are the ones who insist, not the ones who say "no", but this is a context of a such harmless thing like showing your entire face under the mask, that there are chances he is thinking you're doing that on purpose just to play with him, and might feel invited to insist somehow. Especially if you laugh a bit at it instead of straight up seriously making him feel inappropriate.
About the next moves it all depends on what you want: 1) do you want him to stop? Either show your face for a couple of seconds or tell him straight up you're pissed off, unless he stops by himself but it will take longer (well, sooner or later he will stop). 2) Do you want him to continue? Keep playing like you're doing, draw a nose and a mouth on your mask to meme about it and to welcome him playing more. But don't play too long hard to get because at some point he will stop anyway.
Opinion
2Opinion
He is taking baby steps in flirting with you.
ask him why
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