What would you do if another girl kept hitting on your boyfriend?

my boyfriend is a college freshman and I'm a sophomore. he's been at school for about 3 weeks now and made a friend group mixed with girls and boys. one of the girls has taken a liking to him and has made multiple moves on him despite him apparently saying that he's dating me (I know he's for sure said it at least once because I saw him message it to her). at one point they ended up in her room alone which makes me so uncomfortable and I'm honestly upset even though I totally trust my boyfriend. what would you guys do in this situation?