When should we start freaking out about money markets focused on US treasuries (e.g., VUSXX)? What are some other low-risk alternatives?

I have been looking for a house since September and have kept my down payment in a money market fund invested in US treasuries (VUSXX). Of course I'm paying attention to the news and starting to worry a bit about what will happen in the US in the long term. At what point should I consider divesting from those and moving to another low-risk investment? What alternatives should I consider?