Last week I noticed a strange stain appearing in the tile grout in the area of the spa spill-over into the pool. In the past I would see an occasional calcium build up in this area but never anything as depicted in the photo. The color is a reddish-brown. I removed most of it with muriatic acid over a week ago and it's back as of yesterday. Pool has been maintained at balance. This is the only location in the pool I see such an occurrence. Appreciate any input or ideas.