I've been working on an animated music video, and completed it using movie maker. When I completed the video however, I noticed strange pixels in various parts of the video, usually in darker scenes for some reason.

I figured it was due to using a screen recorder that compressed the original resolution, so I redid the video by directly downloading the clips I wanted. The original mp4 files have no such artifacts and its only after adding the videos into Movie Maker that they appear. 

I would like to know what is the cause of this issue, and if it can be fixed?