How can I make molded fake dirt?
Why not use real dirt?
You can buy (or find) potting soil quite easily, which is relatively 'clean'.

An alternative is coffee grounds. To make it as dark as regular soil, you can add a little black ink to it and mix it with water.

In both cases it's best to let it dry first, then mix it with the resin or (maybe slightly watered down) glue inside a Tupperware container or a paper cup, which is both easier to cut away and less wasteful.

The resin or glue has to be waterproof (in case the plant accidentally gets watered), transparent (to let the (fake) dirt steal the show), hard curing (to keep everything properly in place), and preferably matte drying (since glossy dirt will look unrealistically wet or muddy).
Many resins and glues can be used for this - even wood glue, for example, will work fine. Visit your nearest hardware store, and look around or ask someone.