(Just information, not trying to be an alarmist or make you worry about anything )
pretty much all mycelium grows on coco just fine. Lots of speies of mycelium from predatory cobweb molds to magic mushrooms will colonize coir. You can flush oyster mushroom mycelium from nothing but coir. It's far from truly inert, a plant just wont break it down. I put coir in my compost piles fairly frequently because of how much molds and mycelium like it. Helps keep the piles from getting muddy too. Depending on the species of fungi in a bucket of coir, and how long the plant stays in it, ive ended up eventually having to treat coir clone mothers as if they were in soil a time or twice because the root flora was turning it into soil. Higher PH, more infrequent feedings, whole 9 yards lol.
99/100 times you see this, youll see it while waiting for a seed to pop in coir, or while the plant is still small, and its probably just fine. It'll go away as the plant gets established and the water cycling pace quickens most of the time. You can occasionally end up with a predatory cobweb though... Has happened to me a few times.
Something that happens with coco fairly often and most dont actually know what happened:
If youve ever planted an unpopped seed directly into coco, and saw greyish white mycelium like this before the seed ever came up, and the seed never popped, means you had coir likely contaminated with dehydrated predatory cobweb mycelium from time of packaging. Those eat live and dead plant and fungal matter indiscriminately. They like to get into bulk stored coir if the humidity is high enough, because it's viable food for many species of fungi. I was using cheap burpee bricks back in KC for a while, but then the PH on the bricks started bouncing wildly, and several of my bricks came contaminated with predatory cobwebs (assume they switched a supplier to a cheaper one), and i transitioned to DWC because i didnt want to pay any more for coco then those cheap home depot bricks which i hear are just fine again these days
Fun Resulting fact.
*If you pasteurize your coir before using, you will have higher successful germ rates and more control over your root zone flora/fauna from the get go. Less likely to have predatory cobwebs, and less likely to have fungi that'll break the coir itself down into soil. Means overall Less competition, and more symbiosis in the substrate*
If you have fungi growing over the surface of the coir in this manner, even if it's mycorrhizal, and beneficial to the plant overall, that fungi is slowly breaking the coir down into compost (soil). Which isn't a bad thing, but may not be what you want.
