that often happens in good soil. The fungus (commonly called "dog vomit") helps the plants grow better (it is a mycorrhizal fungus), specially solanaceae and cucurbita that you have in the pic. Good soils will have fungi, and poor soils will not. Ants will not harm your lettuce, unless you have cutting ants.

About those ants: Check the lettuce for aphids. They excrete sticky honeydew which ants harvest.
Easy remedy: Hose down the plants every several days.

If the ants or aphids get to be a real problem, you can use nontoxic insecticidal soap. It works dandy on aphids. But don't feel like the garden has to be 100% bug free (or fungus free). It's an ecosystem.

That "dog vomit fungus" is not a fungus but is a slime mold that is not harmful and will go away whether you do something or not. While unsightly there is no real cause for concern. It is also not a mycorrhizal fungus.

http://botit.botany.wisc.edu/toms_fungi/june99.html

kimmq is kimmsr