We don't have synonyms for microplastic yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are fibroplastic, microplasia and microplastics.
(n) :
Small particles of plastic (typically less than 5 mm) produced by the degradation of plastic products, found in high levels in the marine environment and increasingly throughout all environments and in food and drinks.
(n) :
Small particles of plastic (typically less than 5 mm) produced by the degradation of plastic products, found in high levels in the marine environment and increasingly throughout all environments and in food and drinks.