What do you mean your IP address is on someone else wi-fi
Is your computer on someone Wi-fi connection?
If just someone has the same IP address as yours, that is something
different and it doesn't get them any access.
But if your computer is on someone else Wi-fi.
1st lots of system allow the user to get a list of all
ip address activity on the router.
So first, they can easily know you are there.
So if they have their own router, they may be able
run a sniffer program and watch all your traffic.
However, if your traffic is encrypted (secure website, etc),
they can't decode that traffic unless they are trained
hackers who defeat the encryption.
However, you must watch out if some of your activity
is unencrypted and you send the same password
in your unencrypted activity.
Let's say you use an unencrypted file-transfer-program (ftp) or visit
an unsecured website
and use the same password as your password for other accounts.
Then, they can have the password to your other accounts also.