Hmmm... Μιλάω is to speak/talk in general, maybe to another person or even to yourself! Λέω is to say something specifically directed to an audience like tell has to be followed by name/pronoun/intended audience.
Generally, μιλάω = speak, talk and λέω = say, tell.

Edited to add:
See the usage in the following examples Μιλάω ελληνικά = I speak Greek
Μιλάω για τον καιρό = I talk about the weather
Του μιλάω για τον καιρό = I talk to him about the weather
Της λέω τα μυστικά μου = I tell her my secrets
Λέω την προσευχή μου = I say my prayer
Λέω ό,τι θέλω = I say whatever I want (an audience is still implied in the Greek sentence, the intent to communicate something rather than just talk is clear and saying things has an impact because of the implied audience)

Duo considers typos to be correct answers. Although, I wished they'd flag it as yellow instead of green. Because if I see green I just assume it's right, as sometimes, it's not really a typo on my part, it's that I really thought it was spelled a certain way.

Yes

It's the same, because the verb is "μιλάω/μιλώ" ;)

By now you must have figured this out, but for anyone else with the same question: the word for 'girl' in Greek is a neuter noun, not a feminine one.

Duo is learning Greek too!

Really? Well that IS a bit silly, isn't it?