MP4 or MOV or online videos large pixelation playback issues

Hello,

Shortly after I installed VLC, videos I was importing from both my SD card from a DSLR and my iPhone 6s are not playing correctly on my Macbook Air. The preview screens are usually either solid green or gray, I can hear the audio but the graphics are completely pixelated and discolored. Videos before this from the same source still play fine, and the videos also show up fine on a PC. I've tried to use software to fix the videos, but it hasn't worked so I'm not sure if they are corrupt. I've also uninstalled VLC and rebooted the computer to no avail.

Moreover and strangely, some videos online play the same way (large pixelation), but not on YouTube. Just some websites that have video. Please see the attached images for the problem. Image 1 is from an online website with video on the homepage. Image 2 is folder window when I imported the videos into the computer. Image 3 is from Quicktime or VLC when the video plays. Can anyone advise? Is this a software issue? Thanks very much!