- USDT Perpetual uses USDT as collateral.
- Inverse Perpetual (denominated in USD) uses their supported cryptocurrencies (BTC, ETH, EOS and XRP) as collateral. That, of course, involves its own risk since all of them are volatil and can crash at any moment.
You'll need to transfer any of the assets (including USDT) and choose which one to trade. All of them can be exchanged between each other on their exchange.
Also, I'll have to add that you probably aren't 100% safe even if you don't touch USDT. If Tether is confirmed a fraud, what do you think will happen with the price of all the other coins? You can only trade with USDT or other cryptocurrencies (not BUSD or USDC, which are more transparent), so you can't really run from this potential issue.