Sounds familiar.... If I boil or otherwise sterilize wood, it tends to overgrow with fungi. Those range from green to very hairy.
Bark can contain enough food to become overgrown with fungi. And there are always fungi and bacteria on *any* surface, but they keep each other in check.
Once you boil the bark, the balance get's disturbed and one species can colonize the whole surface. At least that's how I see it.
What to do? Clean it at 40-50 degree, so that not all microorganisms get killed, or what I often do: bury/cover the bark in/with balanced soil of compost so that it get's recolonized in a more balance way. I just take some substrate from a roach bin of bury it in there and wait for a week or so.
Another option is to have it dry thoroughly before adding it to the enclosure. Without moisture, these fungi won't grow,