The pictured material looks like an ingredient in the potting mix, even if it wasn't evident when it was first spread. It appears to be either wood fiber (sometimes called "forest products" on the bag's ingredient list) or possibly coconut coir (coconut husk fibers). Wood fiber is becoming much more common in potting mixes as manufacturers try to move away from using as much peat moss, which is less sustainable and often imported. Perhaps the material was exposed as the mix was applied thinly or was wet by rain or watering.

Usually, garden beds are amended with compost or topsoil rather than potting mix, since potting mix contains some ingredients (like perlite and vermiculite) that may break down after only a season or two of weather exposure (especially winter freeze-thaw cycles) or gradually blow away as they rise to the soil surface. Perlite and vermiculite are valued ingredients in containers because they help to maintain drainage or retain moisture, but they don't offer the same degree of those benefits when used outside, so in the future, you may want to use another material as a soil amendment (if amendment is needed).

Miri