We can't be totally sure about your bright yellow 'mold' without seeing a photograph. However, there's a pretty good chance that it is one of the harmless slime molds, very commonly observed on mulch. Slime molds are the source of much concern to those who have never heard of them.

There are many, many species of this very unique organism (which is not a mold or a fungus, by the way). Yellow is one of the more common colors, though I've seen it in bright orange, pink, and white. Pretty cool stuff and it will eventually go away on its own.

Slime molds are not harmful to your plants, and you really shouldn't do anything to contain, remove, or eradicate it. If its appearance really bothers you, simply remove it with a flat shovel and dispose.

Can we see a picture?

If it is what I think it is, it's a fungus that my father and I have always called "Dog Vomit" lol

It happens naturally when you mulch is wet and decomposes

What we've always done is shovel it up and toss it in the compost pile, but if your real afraid of it you could probably spray a diluted bleach solution on it.

Yes, "dog vomit fungus" is a common name for one of the many species of slime mold. Bleach should never be a recommendation for something that is entirely harmless and may even be considered beneficial.