Given the expertise here, I was wondering if anyone else has seen a growth like this at the base of their tomato plants.

This plant in particular is a Valencia tomato being grown in an EarthTainer. In the same container is a Celebrity tomato that is not showing any indication of fungal growth.

My main questions are:

What type of Fungus is this?
Is this fungal growth going to hurt this plant?
Is there a risk of spreading to the other plant and harming it as well?

Thanks in advance for all your help!!