I believe it’s really just like silicate sand. The problem with microplastics isn’t actually the plastics itself. Plastics itself is chemically inert and therefore non-toxic. The problems are:

• Physical: larger junks can cause constipation of the digestive track, which is a problem for some sea animals.
• Chemical: Plastics often have additives that give them special powers, like blocking UV-light. These additives are often toxic, unfortunately. This is why producers nowadays try to minimize the use of additives in plastics. But still, they are there.