Is it a plus point for my PhD application to mention that I am already a PhD candidate at some other university?
I can't really see how it is a plus, but it could be a big negative. 

The obvious question is going to be: why is this student leaving his current university? Doing a PhD involves a pretty hefty investment for both sides, and you're about to leave a university in an attempt to go to another. Universities want to be convinced that you will make it to graduation.

I would expect you to have a very good reason for this (trying to go to a better ranked school after starting at one isn't a good enough rationale to me).