Since this was only used 2 weeks as a QT for new fish, and was not cycled before that - I doubt there is much mulm. It's a thin layer.
I have two other small tanks that I can get seed material from, and I plan to take one of the bio sphere bags out each established filter to put in the new filter.
The tank the gravel is coming from I got to use as a temporary hospital and QT tank. In hindsight, I should not have put gravel in it at all, just left the bottom bare.