We’ve used inspectlet from time to time to help figure out where there are problems in our UI. It tracks mouse movements as part of a complete session. It’s been really helpful.
I'm not so worried about this, as long as it's only on their site and when I'm logged in. They're using it to test features and engagement, and can't really learn much about me as a person with it.
The average user is unaware their mouse is being tracked. Without this knowledge or without given an informed choice, it could be argued that they can't act with full consent. They might be less willing to use a site which does this if they did know.