Yes, it's possible that apps can install a pixel on your store. A link from your store in order to identify the specific line code would help.
Another thought could be to go into your code editor, under the theme.liquid you can do a search using ctrl+F or cmd + F if you are on mac and paste the pixel ID you see. Probably you will find the lines of code which contain this id and you will be able to remove them if you don't want them.
Before you do so, make sure to create a store backup just in case something goes wrong.