I found this while raking leaves out from under a hydrangea. The leaves have been there since fall of course. Reminds me of a 1960's sci-fi movie. Very poofy, broke up easily when I raked it and spread like spores.
You made a good guess. This looks like an old, dried type of slime mold, sometimes called dog vomit fungus, which is what it kind of looks like when it is fresh.
They are harmless.
Here is a page about them: https://extension.umd.edu/learn/fungus-among-us