Normal situation for the wood to have some food of some sort that the fungus will live on for a bit. All the little "critters" in nature have specific times/places when they can prosper and wood newly added is one for this type fungus! Usually not for long and not a hazard for the tank, so I have learned to just let it go. Kind of like snails, algae, or ugly fish? We can fight it and get rid of it in a couple weeks or we can ignore it and it will be gone in a couple weeks!
Plants that die? That's a whole different question and I would look that over and adjust as needed as there are lots of things that could do that. I would not think of any connection between the two questions other than being on the same log except for one question. Is the wood totally dry? It looks pretty fresh/green/new from here. Possible it is a root system recently dug from the yard?