Recently I saw this on bitcoin.org and just downloaded it, what I noticed it's bitcoin address is changing from time to time. Is this a safe wallet like others?
And it's mnemonic is like it's private key?
Like what other guy said above this wallet (greenaddress.com) is one of the secured web wallets unlike from different web wallet out there. You should understand first on how the security works on this wallet, on how to secure your wallet by saving your seed/private keys/backup offline or whatsoever before doing something. The changing of wallet address is for privacy and security reason but you can use the previous address again and it will be reflect your balance on the same wallet.