Probably not working on it during work hours or times when people expect you to work, as well as from not within any company property. If you work on your project on weekends from the comfort of your own house or a coffee shop, that's cool. If you do it during lunchtime at work or on the company office property during weekends, that is a bit shadier, even though realistically there should be no difference. Try to make your personal project situation as clear-cut and separated as possible.