Polkadot.js allows you to interact with different blockchains (Polkadot, Kusama and several others). It is theoretically possible to use one account on different chains, however we do not recommend it as it may lead to confusion. 

We therefore recommend that you restrict your account to only one network. That's why the polkadot.js interface gives you an alert for any account that you have not done this for yet. 

To make sure you're only using your account on the current network, click on the three dots next to your account on the accounts page, and then select "only this network" from the menu:

If you are managing your account with the browser extension, you will not see the option here. In that case, open the extension, 
  1. Click on the three vertical dots next to your account
  2. Open the drop down menu that currently says "Allow use on any chain"
  3. Change it to your desired network

  4. Refresh the accounts page

The warning message will now be gone and your account will show the right adress format for your selected network.

Please let us know below if you found this article helpful or email us at Support if we can be of further assistance.