Until recently the Treasury was draining TGA, which was equivalent to doing QE. In other words, it was spending money which it got earlier from Fed by selling it treasuries. Now Fed does not buy treasuries anymore, so the Treasury has to refill TGA by attracting liquidity from the open market. It's equivalent to QT, which will be added to tightening performed by the Fed. Yes, it will lower money supply, but not without serious risks for the economy.