Its mold. You can still put it in the tank.
When your done with the bottle or if you just want to clean it out and make a new one dump it out and fill it with some super hot tap water and let it sit for a day then rinse it out again untill you get the mold out. You might have to put a stick with some paper towel around it to remove it.
When you remix your next batch add 2 capfulls of excell and you wont have that problem again.
Another suggestion: If you are dosing say pps pro and your only using 5 ml at a time you might cut you csm+b down to 1/3rd and dose 15ml that way your bottle gets used up faster and isn't sitting for so long allowing the mold to grow.