Saltwater, aluminum, and stainless make an nice corrosion marriage, if you are lucky these nuts, bolts and toe rail will come asunder.
When you reinstall them use an anti-corrosion paste. Probably one of the best on the market is T
ef-Gel (amazon link). A little goes a long way. Place it on the threads and under the bolt head, where ever the two metals touch. It doesn't take much. It seems a bit pricey, however it lasts a long time, I've only bought 2 tubs of it in about 15 years. I only bought the second tub (2 oz) after the first one leaped off the boat and tried to swim. Take note, Tef-Gel tubs do not float.
The corrosion in the photo has spread from the bolt, but the cause is the same, SS and aluminum. This bolt may have leaked helping to spread the corrosion. I'd just clean it up, being careful to not damage any of the anodizing that is still there and use Tif-Gel.