Woke up to find this bright yellow, frothy patch on my mulch. It appeared overnight and looks like expanding foam. Any ideas?

you’re unsure whether the organism is indeed slime mold or if it’s growing excessively large and persistent, consulting with a local extension service or a professional gardener can provide clarity.
Additionally, if you notice other issues in your garden, such as plant diseases or unusual pest activity alongside the slime mold, it might be worth getting a professional opinion to ensure that your garden’s health is not compromised.