I have been using GOLDEN Acrylic Paints for about 8 months. I mix the paints in airtight containers with gel medium and usually some water. Some of the mixed paints get moldy after several weeks and often develop a distinctly unpleasant aroma. Why is this happening and will it have a long term effect on the final, dried painted surface?
Tap water contains enough microbes to start the process. We do put biocides into our paints to prevent this growth in our paints, but when the paint gets diluted, they are too weak to be very effective. Believe me, we know the "unpleasant odor" you speak of. Mold and bacteria thrive at a neutral pH, which is what water is usually at. You can adjust pH to be more alkaline by adding small amounts of plain clear household ammonia. A capful into a pint is plenty. This should help prevent the microbial growth.

For the batches you already have, you can add some ammonia to them, and it will kill or slow more growth, but the musty smell will probably remain. Long-term paint effects are minor. Once the water evaporates, the mold can't keep growing on the acrylic paint film. We wouldn't worry too much about it.