Welcome to the site! Organic fertilizer will often show microbial growth as it breaks down - particularly if it is allowed to remain constantly damp. The things that will defeat mold - sun, wind and aridity - are not present in your garage during the winter.
That said, as
@just.wing.it has pointed out, your tree is asleep right now and doesn't need any fertilization. Don't bother to fertilize until you start seeing signs that the tree is waking up in the spring.
When I used to overwinter trees in my garage, I used to place snowballs on all the pots. When the garage would get above freezing, the snow would melt and water the trees. I could easily see (visually) when I needed to replace the snow. Same thing can be done with ice cubes.