"Microplastic" describes a small pellet of plastic less than 5 mm in length. It doesn't refer to any specific type of chemical used to create plastic.

If the question is whether or not lego bricks generate microplastics, the answer is no, not really. Microplastics are mostly a problem when they wind up in the water supply, either from plastic fibers in clothes that are being washed, or plastic microbeads in shampoo that gets rinsed down the drain, or in plastic trash that has wound up in a river or ocean and is deteriorating into smaller particles.