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 the app. The actions, photos and comments that you post in the app are meant to be shared so that we can learn from each other and the app does not currently allow followers to be blocked.
Take the compliment and follow him/her back! You can easily see the activity for all that you Follow in your Following Feed.