SPAXX or FCASH on fidelity as muslim wishing to invest in stocks?

I’m in the fidelity app and am wanting to do personal investments, and the first question it asks is whether I want the core position to be SPAXX or FCASH. I’m trying to be as halal as possible with this, and would love to know if any muslim finance professional would explain which option would be the more sharia compliant option to choose and how to handle or track earnings on interest on this later..thanks!