Can I ask potential advisor what topics he is currently most interested in? I am looking for a topic at the moment, so I'm still searching, and there is one advisor who I'd like to work with. His general topic is also what I want to do in my PhD, but I don't know yet what the exact topic might be. I thought of asking him what he is most interested in at the moment, and see if there might be a fit, or will this shine a bad light on me and my interests?
You could ask, but it would be much better to arm yourself with knowledge to facilitate asking a better question. Do your homework and search for his recent publication record, and find and read the recent results.
So yes, asking "what are you doing now" will reflect somewhat badly on you. Saying "I'm aware of all your published work, and wonder if you are looking into any new directions that aren't published yet" sounds (and is!) much better