These get their name from the white powdery(mealy) wax coating they protect themselves with.
They look like miniature woodlice when you can see beneath this protective covering. They prefer warmer temperatures and are thus more of a threat indoors and in greenhouses.
They will initially be found in joints of leaves etc. If left too long they can form into very large colonies and just like aphids they suck sap and also secrete honeydew which attracts ants and allows sooty mold to flourish.
Wiping away with cotton swabs dipped in rubbing alcohol or methylated spirits works when there are only a couple of individuals. For large colonies insecticide can be used but must be accompanied by a wetting agent to get into their waxy protective coat.
They, just like aphids, have also been around for millennia and are found encased in amber... this one is being carried by a winged ant