If you see matches from other teams in your activity feed, it means you follow a Mingle user that is part of one of those teams.
When you follow other users in the Mingle app (and they accept your follow request), their activities become visible to you.
You will for example be able to see their upcoming matches, follow their match feeds, and vote for the Players of the Match once their matches are finished.
To check who you're following, go to your profile and click on 'following'. The matches of the Mingle users you follow will automatically show up on your match feed.
To stop other users' matches showing up on your activity feed, you have to unfollow the Mingle user.
If you believe you are wrongly seeing other team's matches show up on your activity feed (e.g. you're not following a member of that team), please get in touch with support.