Marcite Pool - Brown "area" discolored

[hagotime]

I've had a brown stain develop over the winter that I could not managed to brush away at all. It has covered a fairly large area. The pool is finally large enough to get in so I went under with goggled to look at it. The area makes a slight divot into the marcite and it really just appears like brown plaster vice a stain of some sort. The texture is not quite like the marcite. No clue if its a growth or whatever is below the marcite. It doesn't go very deep but it is slightly below the rest of the pool area. The area is a little bit smaller than the size of my hand. I'm concerned that A. Its not going away, and B. If its going to spread or majorly damage the pool. Thanks. I posed a zoomed in picture and normal view from above.

 

[Swampwoman]

Have you tried the "vitamin c test" to determine if it's metal-based? If not, crush up some vitamin c in a sock and set it on the stain.

It would be helpful if your specs were in your signature and if you posted your water parameters.
Thanks