How do I stop Safari from occasionally downloading "mlingos9.html"?
It's a problem with a specific web page — this one.

It specifies the content type application/octet-stream in the HTTP response headers, which is usually used for files meant to be downloaded — and that's what your web browser does. HTML web pages usually use text/html or similar HTTP headers, never application/octet-stream.

If you were browsing web sites with similar topics that might link to this web page, and you clicked a link, or the web page was included in another web page using a frame/iframe, that's the cause for the downloads.