It's not everything like microplastics. It's everything like everything. Palmitic acid is one of the most common fatty acids. Cocoa butter is 25% PA. Cow milk is 30% PA. Chicken is 20% PA. In human milk, 25% of its fat content is PA. Olive oil has tons of it too, so do soy oil and sunflower oil (and probably any vegetable oil you've eaten). But if you don't eat anything at all, human body is still about 2% PA - as I said, it's a very common chemical compound.

To quote Wikipedia:

Palmitic acid, or hexadecanoic acid in IUPAC nomenclature, is the most common saturated fatty acid found in animals.

If it were dangerous for animals, there would be no animals left living. It's not like microplastics that are exogenous. It's part of what living organisms are made of.

Instead, what happened is probably some kind of p-hacking - they gave this compound, lots of it to be sure, to lots of poor mice to which they gave cancer previously, and in some of them the cancer got worse, because it's cancer. So they wrote down which cancers those mice had, and declared that they linked the compound to "spread of cancer in mice". Easy peasy.

Sources: https://www.sciencedirect.com/science/article/abs/pii/S03088... https://www.sciencedirect.com/science/article/pii/S095232781... https://orbi.uliege.be/bitstream/2268/240227/1/fatty-acids-c... https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5682332/