You just need to drop below the minimum and it will stop. You wouldn't do anything from the mobile wallet observe you would do it from veforge. Also you can't but an xnode from a ledger wallet. You would need to put the VET into an account using the mobile wallet, buy the x-node from there and then you can transfer the x-node to a ledger run wallet. After you transfer the x-node in you can't transfer it out though as the same limitation with veforge makes it impossible. I have heard about a fix forever but still to not yet, so make sure you are getting the x-node for the long haul or keep it on a wallet formed with the mobile wallet if you're looking to ever sell it.