You may not be acquainted with the person that's Following you, but he/she is apparently inspired by your sustainable actions and wants to Follow your activity within the app.
Social is an important part of what we do. The actions, photos and comments that you post in the app are meant to be shared so that we can learn from each other.
Take the compliment and follow him/her back! You can easily see the activity for all that your Follow in your Following Feed.