I haven't seen the stuff but it sounds like a type of algae. If so, it's probably not coming from the hatchery directly. The nutrients released from a hatchery can cause algae growth downstream, particularly if the water is clear and/or warm. I'm not sure if this is occurring on the Cowlitz but it sounds suspicious. WDFW/Tacoma Power could be cited if they've gone over their NPDES (discharge) limit for nutrients (nitrogen, phosphorus).

But this is all speculation on my part.