GIF export creating white dots?

I'm exporting GIFs for my website. They have to be under 20 MB in size (squarespace limitation). I am getting "white dots" on my exported GIFs. Curious to understand what this is, what is causing the issue (compression?), and possible fixes? The answer is not to make the file bigger. I am restricted to 20 MB file sizes. Screensbots below are from 16.8 MB size GIF. Thank you.