Consider how the wizarding world is hidden from the muggle world in the Harry Potter/ Wizarding World universe. Or in Hell Boy II. The troll market is hidden under a bridge. Red can see it but it's veiled from the human eye. Shadow Hunters. Clary can see the runes outside of the nightclub but Simon can't see them. I have been referring to it as a troll market concept but I feel like there is a better word(s) for it.
I might be missing something here, but this is describing an "invisible" or "hidden" world, where "invisible/hidden" is usually qualified by such phrases as "to the normal eye/from the normal eye" or "to/from those who do not have the power"; "to/from all but the chosen few", etc. or "except on midsummer's eve" etc...
invisible and hidden have their normal meanings.
The following term might be used, as, the most often, those more or less hidden entities are considered to be unacceptable from the point of view of decent society, or barely acceptable, or at times, just not conferring the idea of people having a pleasant station in life.
(SOED) underworld 4 A sphere or region (regarded as) lying below the ordinary one. Also, a lower, or the lowest, stratum of society.
• T. BULLEN The begrimed … toilers … in the underworld of engines and boilers.
• attrib.: San Diego A playwright … solicits underworld figures to back its latest play.