Mold dungeons are a sub-biome of the Azeos' Wilderness. Everything here has been overrun with mold, including the zombie-like Infected Cavelings. These slow moving mobs can poison players with their melee lunge. They are dangerous in large numbers, small spaces or if a Mold Tentacle lands a sneaky projectile hit, with its hard stun.
The Mold Ground itself spawns both mob types and inflicts a debuff that slows players (and non-resident mobs) by up to 75%, at 10 stacks. The mold water will infect the regular Wilderness water, on contact, changing its fishing loot table.
This biome is arguably still the hardest area in Core Keeper. With few rewards to be found inside, aside from the mob's drops; Mold Vein Necklace and Mold Ring negate the mold debuff, when worn as a pair.
For raiding Mold dungeons, it is highly recommended to wear both the Mold Vein Necklace and at least one Mold Ring. Together these give a set bonus that negates the up to 75% slow from the Fungal Infection debuff.
Given that these are both drops from Mold mobs, initial forays are much harder, and fighting (or mob farming) should perhaps be limited to the periphery of Mold biomes. Although, speed buff items and foods may assist, while cooked food which includes the (easily caught) Rot Fish will give 30 seconds of immunity to Mold infection.
Carefully clear out chambers as you go, using a ranged weapon. A Fireball Staff at minimum. Look out for Mold Tentacles which glow. Their ranged attack projectile is hard to see and will hard stun players it hits, giving time for the zombie-like Infected to close distance and finish the job. Hence, the Escape artist Running talent is also highly recommended, to reduce the duration of these stuns. A useful off-hand item for the exploration is the Swift Feather, that allows you to dash, a very useful movement to run from the monsters.
These mobs have a significant health and spawn multiple at a time. So chewing through a dungeon can take a long time, which risks spawning more mobs behind, in the mean time. So it's recommended to block further spawns by hoeing the Mold Ground. That requires painstaking breaking all the mold veins on the ground. So, slightly quicker alternative is laying down floor tiles instead, such as Wood Floor.