Stain on Gunite Pool Surface

I have a gunnite pool, 15000 gallons, salt water.

I have a pretty nasty stain on the floor of the pool. It appears to be the result of something metallic that reacted to the salt water. The stain is about 3 inches long and spread out about the size of a human foot...(best description I can provide!). I have a few photos of it if that will help paint a picture of what I'm dealing with.

Any ideas on something I can use to remove this stain, without having to drain the pool?

Whatever it was that caused this has apparently deteriorated and dissolved. I've tried scrubbing it away with brushes, to no avail.

Try holding a Vitamin C tablet on the area and see if that removes it.