The problem is that even though you are making new batches the mold, fungus and bacteria are still in the bottle and are carried over from batch to batch. This motley crew of nastys are being sprayed in your hair which can cause itchy scalp and dandruff.
Sanitize your bottle in between batches with a 5% bleach solution. Let it sit with the sprayer in the bottle for 5 to 10 minutes making sure to spray some through then thoroughly rinse. And keep your spray mixes is the refrigerator. I agree with the previous poster; I would see about adding a natural preservative and an antiseptic like essential tea tree oil. Try using distilled water. It will have a lower bacterial load than tap water so it'll take longer for the bacteria to proliferate to numbers shown in your picture.
This is one reason why I don't make homemade spray mixes myself. I find a good cheap water based leave in conditioner, Infusium 23, and leave the science at work lol!
(I'm a biological chemist)
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