+1 to the notion that skimmer gunk is way to salty to use on plants. The water component of the gunk in the skimmer cup will be the same specific gravity as the water in your tank, which is many, many times more salty than any land plant not adapted to an alkali or salt slew can handle long term.

Theoretically you can remove the salts, but doing that in a way that does not also remove most of what you're trying to save in the gunk in an efficient and speedy manner would require equipment you probably wouldn't have or want to invest in (like a centrifuge).

Though perhaps I'm thinking too much like a scientist, Maybe you could try collecting it in coffee filters (though the pore size of the filters might be too high for the super fine material in skimmate) and rinse it thoroughly until the salts have all been leached. If you bought the right coffee filter could then compost the skimmate and the filter together. It might take longer than it's worth for what you'd get out of it though.