Just like the name says these are mites, spiderlike and can often be red.
Aside from red they can be many other colors from light yellow to green to orange in summer but they are commonly red in winter.
They sometimes make webs and here is where the spideryness comes in.
they are not easy to spot since they are so small and so are most often detected by the damage they do or the fine webbed masses they create. Since the pierce and suck sap from leaves the leaves begin to show fine mottling and other discoloration that may eventually turn the entire leaf blotched and yellow.
Misting plants can prevent infestations or keep numbers low since they cannot handle high levels of humidity.
They can also be removed by spraying with water. There are also chemical remedies usually in the form of insecticide sprays for severe infestations.