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They prefer a pH between 6-7.

The powdery substance could be mildew or some sort of mold or fungus attributed to overwatering. However, it could also be due to salt deposits. Allow the plants to dry out some and try scraping the substance away with a spoon.

Lime trees, whether in pots or the ground, typically require a nitrogen-rich fertilizer. Here is an article that you may find helpful: https://www.gardeningknowhow.com/edible/fruits/citrus/fertilizing-citrus-trees-best-practices-for-citrus-fertilizing.htm

Try treating the plant with neem oil instead. This article will give more info: https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm

Limes do not turn green. They start out that way and turn yellow as they ripen. Limes are actually taken prior to their turning yellow. If your limes remained yellow, either they were left on the tree too long or you have a lemon tree rather than a lime tree.

This articles will also help: https://www.gardeningknowhow.com/edible/fruits/lime/is-lime-fruit-and-lime-blossoms-falling-off-tree-normal.htm
https://www.gardeningknowhow.com/edible/fruits/lime/lime-tree-harvest-time-when-to-pick-a-lime-from-a-tree.htm

This article should help: https://www.gardeningknowhow.com/plant-problems/environmental/learn-how-to-avoid-and-repair-transplant-shock-in-plants.htm