My boyfriend’s ex gf followed me on IG

She followed me with a fake account. I was curious to see who would be interested to see what I’m up to because I’m a pretty lowkey person who hasn’t got lots of known friends. I went through the account’s following and they were 80% celebs and the rest are all girls that her current boyfriend follows + me.

How should I feel about this? What should I do? Should I like let her know that I know? Lol