If you're talking about an "earth" stock tank, the scum (algae) is related to algae spores and weather/temperature. Need to either aeriate with a fountain spray or put in copper sulfate or some of that very dark blue algaecide colorant--available as either concentrated liquid or crystals.
If you're talking about a "stock tank", then drain it, wash it out, fill with fresh water (as often as needed) and add some of the blue colorant to fresh water.
The blue colorant is ok for livestock (according to the directions on the package) and, it hasn't hurt our livestock any.