You may have already noticed it: a yellow yellow -like liquid on top of your healthy mycelium. What exactly is it and what can or should you do against it? We call them metabolites. Some label it as an infection of substrate while others are convinced that it is a natural defense mechanism of Mycelium, similar to white blood cells in a person. The latter is the most obvious theory.
Metabolites are observed the most when growing on coffee grounds. Not entirely coincidentally, this is a breeding method with the best chance of infection, because coffee grounds are never really sterile. The success with this breeding method is mainly due to the enormous power of (gray) oyster mushrooms. These are so aggressive that they grow almost everywhere. When the Mycelium notes a potential infection, the metabolites will separate to tackle it. So super handy stuff, that yellow liquid on your brood!
What should you do about it? As far as we are concerned, nothing at all, because removing the natural defense mechanism from mushrooms is not that handy. Just like humans, Mycelium is also an intelligent "orphan" that regulates many things. What you have to look after is the fuse of your substrate. When an infection attacks the Mycelium, stationary water is the perfect breeding ground for that infection. Ensure a very nice fog to keep the mycelium moist, but don't let it become water. You run the chance that your powerful mycelium will still take it against the infection with all its consequences.