Orange sponge-like mold growing on mulch - what is it and is it harmful?

Question

There is a mold that looks sponge-like that is growing in various spots on my mulch. It seems to appear very quickly (it seems like overnight but that's probably because I'm not observing daily), and I notice the following life cycle:

1. The mold is a moderately rich orange color that looks neither dry nor moist.
2. The mold seems to rise (like baking bread) and expand in diameter and turn a more pale orange (like in the first image below).
3. The mold "pops open" and exposes black spores (like in the second image below).

I can poke the mold at steps #1 or #2 above to make it "pop" and have the black spore exposed.

The mulch was made from some trees that I had cut down on my property, one fir and one white birch.

I am wondering a) what is this mold and b) is it benign or could it be harmful to plants?

Answer 1

Fairly sure you got slime mould Fuligo septica, commonly called dog vomit slime mould. Its substrate (what it grows on) is generally wood chips. It's not a fungus though as slime moulds fit into an entirely different taxonomic group. Here's a photo of dog vomit slime mould that looks very much like yours.

They often show up after a heavy rain or if you watered the area. It's completely harmless but if you'd rather not have it in your garden, rake and break it up or scoop and dispose of it is all you need to do.

Slime mould are actually quite fascinating!