Is the fuzzy stuff under the seed mould or early signs of roots?

Mold, but I've had them grow a fair amount of mold before the roots come out. Don't despair!

It's mold. Do a water change and turn to your old friend Hydrogen Peroxide.

Thanks, I might get some more avocados & start again. This pit came from an avocado that was kept at a surprisingly low temperature in the shop

Good experiment Bg! Squirted some in & I could see the mould disappearing

Mould has completely gone now!