Probably one of the slime molds. I suggest you post a picture in the Garden Clinic Forum just to be sure.

Agree that it's probably a mold ~ some of them are really yucky to see. You can get rid of most garden molds by wetting them with soapy dishwater. About a 1/2-teaspoon liquid ivory or other very mild soap per gallon of water; stir, don't shake. I haven't met any plants that objected to the soapy water, but pour a bit on a leaf and wait a day to see if it wilts, withers, yellows or otherwise seems horrified by the bath. If plant ignores soapy, then thoroughly wet mold and the area around it.

Do you use bark as mulch?
I always get a bunch of mold when I do.
When I put water on it, it has a dry brownish powder
that goes every where.

Meldy, I'll try the soap next year.

It's worth mentioning that the term 'slime mold ' is a misnomer. These yukky things aren't molds at all. They aren't a form of fungi, either. They pose zero potential for harm to us, our plants, the soil, or anything else.

Now, I'm assuming that this stuff is growing in the mulch. Slime molds can climb on anything in the way....plants, wooden fences, brick, cement....anything. I wouldn't bother using anything to get rid of a slime mold....they are a beneficial part of earth's recycling team.