BB, as you have a 'plaster' pool, you can place a trichlor tab on the stain and see if it goes away (a satchel of cal hypo will do the same thing). If you get no lessening of the stain, you can put a satchel of ascorbic acid on it to make it go away. (this is some quick info, for more look at the 'stains' section of the forum)