Not a chemist, but plastic is just a general category of “lots of molecules linked together” (polymers). Plant cellulose is technically a polymer, so is DNA, but colloquially plastic tends to refer to ones specifically made industrially from fossil oil, like PVC, ABS and so on. Xanthan gum is made by fermenting sugars and linking them together, so it is technically a polymer, but not what people tend to refer to when talking about plastic. Idk about microplastic effects, but it is pretty well biodegradable and considered safe according to the internet. I wouldn’t worry about that, not any more than about other health risks like charring, red meat, sugar, alcohol, regular old microplastics…