Does happo ike open in early march? Or do you recommend any other place for hinking or sightseeing (like kamikochi)?
March is still snowy in Nagano, so hiking trails including Happo Ike will still be covered in snow. While most hiking areas will be closed during winter, you can enjoy a similar experience by trekking around snowy fields while wearing snowshoes. Take a look at this article for more information about places where you can snowshoe, as well as other winter activities.
I hope that helps answer your question.