Hello and welcome! 
It looks like Micro.blog currently lacks support for this. The mention in your post gets confused for a Micro.blog username resulting in the broken link. That’s something @manton should look into and probably wants to fix.
Until that happens, there’s a workaround. You can manually edit your post and replace the mention with a working link using Markdown syntax:
[@costanza@micro.blog](https://micro.blog/costanza?remote_follow=1)
Which will end up like this: @costanza@micro.blog.