It's useless to strain it, because some mold will always penetrate the strain's holes.
You may use the solution to water your non-hydroponic plants, grass, flowers or some trees.

Always clean your bucket with bleach before filling it with the stock solution.

Put your solution always away from sunlight. Mold grows with sunlight, in dark conditions it can't grow.

It's better to prepare 1 - 2 liter of concentrated solution. Five gallons is too much, unless you have a lot of plants.

If you can, use distilled water to prepare the concentrated solution.

You don't need to destroy your plants, try washing the entire system.

And check the pH of your concentrated solution, it should be acid.

effie4050
First are you sure it's a mold, and not just residue from the dry mix? There is often a powdery residue left when mixing dry nutrients.

Second, that is a mold it will be extremely hard to get rid of because just one spore starts it all over again. Also mold does not need light to grow, I'm sure jamesvlad was thinking of algae winch does need light. It's always recommended to keep your nutrient concentrate (even solution) in a cool dark location for that very reason.

(1) If it is just a residue left from mixing, then it wont mater. Though it would indicate your nutrients didn't mix correctly.

(2) If it is alage, then just skimming it off, and keeping it in the dark is all you'll need to do.

(3) If it is a mold, you'll likely never be able to get rid of it from your concentrate. I cant really think of a safe way to even try unless you tried mixing in about 1-2 cups of 3% hydrogen peroxide (H2O2) per gallon. I use it a lot in the nutrient solution itself, but have never used it in a nutrient concentrate. I just don't have any idea if it will affect the mineral elements in the nutrients being so concentrated. If it is a mold, and in your concentrate, you'll just keep spreading it to your plants every time you use it.

If you aren't sure, I would recommend to take some pictures of it, then e-mail them to the nutrient manufacture to see what they think. Even if they cant tell you for sure what it is or how to fix it, they should at least be able to tell you if it's normal or not.

Also this may be obvious, and I know you said you mixed per instructions. But are you sure? Generally nutrient concentrates need to be mixed into separate parts. Most one part (complete) dry mixes shouldn't be mixed as a liquid concentrate.

Also, remember most molds will not harm a person. That green mold that grows on bread? It may not be pretty. It might not taste good. But, it won't hurt you to eat it. Same thing applies with most molds (like 99% of them) Not to mention its in the water which you're not going to drink anyway and it unlikely to cross into the plant material so you needn't be too concerned with it.