Advice needed--reaching out to a potential advisor
Of course you should contact him.  There is no harm in that.  But I wouldn’t mention professor A.  Write a simple and short email stating the you have already applied in the department and you are looking for a supervisor for your PhD. Then also mention that you have been reading papers on topic X and you find them interesting. And since his research interests are similar to what you want to do, would he be interested in serving as your supervisor?  And then end your email by asking him to review your application and contact you in case of any question or query.

Good luck!