How about unresolvable (or non-resolvable), derivatives of resolution? From Cambridge:
resolution: the ability of a microscope, or a television or computer screen, to show things clearly and with a lot of detail
Physical scientists apply this word to imaging instruments/devices/techniques, including the human eye, microscopes, magnifying glasses, telescopes, etc., that cover length scales from the smallest (e.g., scanning tunneling microscopes) to the largest (e.g., giant telescopes). They often speak of the ability of a given technique to resolve features at a given scale, or, similarly, of features at a given scale to be resolvable -- or not, i.e., unresolvable or non-resolvable, beyond the resolution of the given instrument/device/technique.
The mites that cause sarcoptic mange are beyond the resolution of the human eye.
The human eye is not capable of resolving the mites that cause sarcoptic mange.
Are the mites that cause sarcoptic mange resolvable by the human eye? No, they are unresolvable (or non-resolvable).
You will not find these senses in English language dictionaries.