I think hobby itself would do just fine. Hobbyists work at their craft when it’s fun and convenient, but don’t make the necessary commitment needed to get to the professional level.
"Selling apps on Play Store is my hobby".
So you may be a "hobbyist programmer ". Some people actually hate that name.
a hobbyist programmer is somebody who spends 10 or more hours a month programming, but is not paid primarily to be a programmer.
What you're describing may also be a sideline career.
An activity done in addition to one’s main job, especially to earn extra income
[As modifier]: a sideline career as a stand-up comic
"He founded the fast-food company as a sideline to his petrol station"