Do I have gynecomastia or pseudo gynecomastia? The skin under the nipple is a little hard. (Photos)

Gynaecomastia is a condition that affects males when they get breast gland development under the nipple. It can be very small or minimal growth, called grade I or fairly large and ptotic called Grade IV.
You seem to have a very small gland under the nipple. It is causing the nipple to protrude. It is also called puffy nipple syndrome. It could be classed as Grade I. As you are lean, there is not a lot of fat in the gland. A minimal liposuction around the edges could be done to leave only fibrous gland before excising the gland. Or simply through a tiny access sub nipple incision, the gland can be excised. That should cure you permanently.

You have a clear case of typical gynecomastia. If it bothers you, obviously it does you can seek its surgical removal.