This pest appeared on the squash plants yesterday. What is it? How do I treat it? Zone 6 near Boston, Massachusetts.
This is another case of "dog vomit slime mold", or Fuligo septica.
It pops up unannounced, often on wood chips (which I see isn't your case here), and spreads a little bit, but in sort of a limited way. By that I mean it gets a little bigger, but doesn't really spread past the immediate area that it started in. It goes through a definite short lifecycle, changing color from psychedelic-yellow, to vomitty pinkish-beige, to black, all in the space of ... a month I guess(?), and then it dries up and blows away. Unless it engulfs your entire squash plant (and it doesn't look like it is huge enough to do anything close to that), it won't really harm anything.
For a really interesting nature lesson, you ought to look up this disgusting looking, but fascinating organism on the internet. I think most people don't feel it needs any treatment, other than just to stand back and admire its performance (maybe take some photos, and share with young people.)