These types of fog are gameplay features intended to obscure the player's vision in survival mode when in a certain block. Requests to disable these in the mod have been rejected due to these being a survival gameplay change, however they still appear in Spectator, which is counterproductive as this game mode is intended to give clear vision in all cases. As such I'd recommend making it not show up in this case.

It is worth noting that while the fog still appears in Spectator, it is generally in a greatly reduced form (see the fixes to MC-71530 and MC-213743), specifically due to the fact that they shouldn't be a nuisance in Spectator.

Vanilla bugs MC-214186, MC-219893 and MC-219894.