This is most likely underexposure, not a special film defect. With color negative film, a very thin underexposed negative can still be printed by the lab, but the correction needed often produces muddy color, low contrast, and a greenish fog/cast.

Because it only happens on some frames, the most likely cause is inconsistent exposure rather than every roll being bad. Try giving the film more light: use a slower shutter only when safe from blur, open the aperture more, or shoot in brighter light. If you’re metering manually, double-check the meter reading and your ISO setting.

Color negative film is fairly forgiving, but there’s a limit. Once a frame is badly underexposed, the lab can’t fully recover clean color and density. If future rolls exposed a bit more generously look normal, that confirms the issue.