I thought it was a heavy water bug. But the definition of Concha Utrillas Gómez fits well with the result of the decanted raw water.
The correct term is pooled water or better still stagnant, with Z (unless it refers to Posada as a brand or surname). Pooled water is the same as stagnant water, retained water or stagnant water.
Water Inn: It is the water that in a container is allowed to rest, to be still, so that little by little the products that are dissolved in it are held in the bottom and the water is transparent after a while.