[Not sure if this belongs in this topic, this is as close as I could come to a "cleaning" topic ]

All,

New ptec (about a month old). Recently I noticed a whiteish stain of some kind on the pool floor, in the deep end. Diving down with goggles, an up close inspection revealed that there is some kind of white residue clinging in between the pebbles, and it won't brush off with a brush. I'm thinking is some kind of scale, maybe from chemicals dumped into the water? Anyhow, I was thinking maybe a string acid wash would get rid of it (it's only about 1-2 sq ft. in size). But how to apply the acid underwater? I don't want to bathe the entire pool in acid just to remove a small stain. Any tricks for doing this? I am leery of diving down with acid, letting it go in the pool, and trying to scrub. The acid would surely surround me quickly :-/