A Single Word for "Secretly watching your crush"
Solution 1:
I think furtive/furtively works here.
behaving in a way that shows that you want to keep something secret and do not want to be noticed
- He looked at her furtively
- He cast a furtive glance towards her.
It doesn't have the negative connotations of spying or stalking.
Solution 2:
We also say "to steal a glance".