Rizz up means to flirt with charisma to attract someone romantically. "He rizzed her up" = he flirted skillfully and got her attention. Slight evolution of "rizz." Not concerning, just teen flirting vocabulary. If your kid uses it constantly, ask them where they first heard it, not to police their speech but to understand which online community they are in. Most slang of this kind moves through TikTok and group chats, so seeing it in your child's vocabulary just means they are plugged into normal teen internet culture.