I read several post on this forum re the importance of mulm in a newly set up planted tank. I'm moving in 10 days and I collected some mulm from a tank. Do I have to aerate the mulm? Can I just put it in a closed water container?
Generally if possible, use it as fresh as you can, if not, a bag is fine.
There's not much food for the bacteria, but it'll bounch back fast, just like the sealed stuff that they sell like Bacteria Cycle etc, but it'll do better because unlike the Cycle, it has the specific bacterial decomposers that break down plant waste/detritus, not just fish waste/NH4.
Bacterial plant decayed, fish waste detritus might be a more jargon type word for it. Loaded with organic matter, bacteria and fungi, as well as a source of food for them till that acculumates in the new tank.
It's active, living and adds precisely what's missing from a established tank to a new one.