This appears to be either a slime mold or a fungus that is beginning to decay the mulch -- both are normal and to be expected, and both will go away in time, especially during dry weather with no irrigation. Neither of these organisms bother live plants, though they may consume plant debris that is already dead. You don't need to do anything about it, and no treatment is recommended (or effective in this case).

If you can show us photos of the symptoms you're seeing on the garden plants, we can try to diagnose them. With young plantings, often it's just environmental stress (too much water, too little water, etc.) that causes damage or dieback, though minor diseases like gray mold (Botrytis) or powdery mildew can appear when the weather conditions are ideal for those fungi.

Are you irrigating the planting bed regularly, or only when the soil feels dry? (Or are the plants relying on rainfall?) Ideally, you want to periodically probe the soil a few inches deep and only water when it is beginning to feel relatively dry to the touch; if it feels damp, the plants probably do not need watering. As a very rough guideline, most garden plants would only need a good drenching around once a week or so during hot, dry weather. This interval can of course vary with soil type, sun exposure, and plant preferences. Damp conditions from irrigation (and factors you can't avoid, like dew, rain, or very high humidity) can allow for spores of fungi and bacteria to spread and infect leaves more easily than if those surfaces were dry.

Miri