The chalky powder is the zinc breaking down in the aluminum alloy. The latter reduces weight. Most trailers run on tandem axles. Suspension is either leaf spring, torsion, or air. The leaf spring is a mechanical suspension.
Springs bend to absorb shock, and are subject to corrosion and wear.
The torsion suspension is a rubber-cushioned axle assembly. The torsion bar, enclosed in rubber inside a steel tube, gives the trailer an independent suspension on each wheel. There's no way to determine the condition of the rubber until you have a failure. Some high-end manufacturers offer air suspension on horse trailers. This option increases the trailer's original cost, but delivers a smoother ride through air-filled rubber bags.