Does your photo say "Polarr" in the bottom right? That's the default watermark! And you can easily remove it yourself.
Just go to export/save and then tap the watermark tab. On that page, you can toggle on and off the 'use watermark' switch. That will remove the watermark.k
Want to use your own custom watermark? You can upload your own transparent PNG file with your logo to use as your own custom watermark.