That's probably coming from the title, not the alt text, although I can't replicate the behavior you're seeing. The browsers probably aren't handling blank title attributes well, or perhaps they just have a single space which is showing the title popup on mouseover.
I would recommend just putting in a title on the custom link that describes what the link does, since that's good for SEO anyway. I also fixed the alt tag to replicate the title in the version I'm working on that merges the temporary plugin into the main one. Of course, removing the title and alt text should prevent what you're seeing from coming up too, although they're there for a reason.