There is no established "definition" for micro beads but they have been claimed as ingredients in a number of products for decades. As a general rule, this term is used only for products that are milled, ground, chopped, or otherwise mechanically reduced to "powder" from solid materials, which may be "natural products" like apricot pits or mica, or "manufactured materials" like a variety of plastic polymers.

There are practical limits to how finely a solid material can be "ground" so the smallest of these might clog sweat pores, but are not generally at "molecular sizes."

A newer class of materials, often called "nano particles" has appeared, and at least a few cosmetic preparations have claimed to have included some of these. Particles of this kind are usually made by "building up from the molecules" by chemical methods, or by vaporizing and condensing solid materials.

Many of these nanoparticles can be made small enough to essentially "act as molecules," and in medical uses have been shown to be capable of passing through cell stoma to enter individual cells, where - in the cosmetic case - their potential effects are almost completely unknown.

As long as no specific medical claims are made, in the US there is no agency having authority to regulate either the composition or advertising claims of such products, so the consumers have no way of knowing what such stuff is going to do to them - other than what the advertising says. Even the distinction between micro- (usually not biologically active) and nano- (quite likely to affect body functions?) cannot be trusted, since advertisers use the two terms pretty much interchangeably depending on what they think will get their junk sold.

The rush to claim "nano-" stuff because it "sounds modern" is, IMO, exceedingly dangerous; but the crap that's only "micro-" on the label may actually be "nano-."

All that can be suggested is that you DO CHECK THE INGREDIENTS on the products you consider using, and believe a little less than half of it. And remember that the more extravagent the claims, the less likely you'll want to put it on (which may mean IN) your body.

John