In the atlas project why the tor-relays ip are discovered? isn't it
dangerous?
There is nothing strange or dangerous about that. After all your ip has to be published somewhere otherwise no one would be able to use your relay. You could set up a bridge relay. in that case, your ip isn't listed in the complete tor directory - but still accessible, check the docs (https://www.torproject.org/docs/bridges.html.en)
Security is gained by using a firewall, and having an up-to-date-system.