As other people have said, oozing. In addition to their fixes I would suggest looking into sequential printing as a print method to be aware of. Rather than printing two objects layer-by-layer, sequential printing will complete all the layers of one object before starting on the next. This has the major benefit that you get zero oozing or stringing between parts. This can be a lifesaver on materials like PETG that will always string to some extent. The only gotcha is that you have to place the parts further apart on the print bed, as the extruder will collide with any nearby completed parts (Prusa slicer has all of this built in, including alerting you if any collisions would occur). I don’t recommend it for every print, but it’s a useful tool to have in your toolbox.