If the mnemonic phrase gets exposed, all of the accounts associated with it are exposed.
To get a new mnemonic phrase, without this app, you have to
- write down the mnemonic phrase (or all the private keys of each account you have created)
- reset the app (deletes all the data)
- generate a new mnemonic phrase
- import your accounts (MetaMask allows importation by private key only)
WARNING: keep your mnemonic phrase and private keys very secure.