My advisor used my research idea and wrote a paper with another student, what should I do?
It's not clear whether you are still in the same research group. I suspect not, or you would have been aware that another person in the group was doing the work and paper. While it is morally questionable to use someone else's idea without asking them to be involved, there's nothing you can do in the sense of 'getting it fixed'.

But it may be worthwhile sending an email that is not accusing. The adviser may have felt that you were fully occupied, or that the project was too easy for you but was a good training project for a student, or simply forgotten about the conversation or any number of other reasons. An email like:

Dear Prof X - I saw your recent paper Y. As you may remember from our discussion (approximate time frame), I am interested in this idea and the paper looks very similar to what we discussed. I am disappointed not to be involved in the implementation so far and would like to participate in any future work coming out of this paper.