That's Aluminum Chloride (AlCl3) from our water tank. In addition, there was sand. We had noticed the particulate in our drinking water, so we knew the tank needed to be cleaned.
The good news is that we have to gloriously large inspection ports , making cleaning possible. The ancient gaskets had to be pried apart with a crowbar. We replaced them with hand-made gaskets cut from PVC shower-pan liner.
CA scooped out a great deal of sand and aluminum chloride. We tried using a Jabsco drill pump to extract the last of the water. However, the sand eventually abraded the interior of the pump, destroying it.
We replaced the (dead) drill pump with a manual pump. Wow! This manual "oil change pump" rocks. The drill pump is a two person job: one person to hold the hose in the tank, the other person to operate the drill and guide the hose into the bucket. The manual oil-change pump is a one person job: guide the pickup with one hand, pump with the other. And the gallons per labor hour seems comparable to the drill pump.
In addition to sand and aluminum chloride—now removed—we have some old PVC gasket material that was used to seal the tank and baffles. This has gotten crumbly. CA pulled out the loose bits. We'll probably install a filter to catch anything remaining bits.
Finally, she washed the entire inside with vinegar water. It's a long, long reach to the forward edge of the tank. It's hard to see in the picture, but the left-side (aft) inspection plate is surrounded by the bottom of the settee. That's a couch-height wooden box. She has to lean down into that box to be able to lean down into the water tank.
"Yoga helps," she said, "a lot."
The port tank is now sparkling clean and waiting for water. We still have to do the same job on the starboard tank. But, we now have the right pump. There's no settee on the starboard side, making it much, much easier. Finally, we've done one, so we know what to expect.