Hi Joe!

Your dad's got good old-fashioned blanket weed there. A variant of hair algae, it thrives on mineral-rich water, fish poop (ammonia), and sun. This being Texas, I figure he's got an abundance of all three.

Short-term, he'll do just fine with a biff brush and soft squeegee. Long-term, adding aquatic and verge plants to the environment and upgrading his filtration to get the ammonia cleared more efficiently will work better.

Algae thrive on ammonia, NOT nitrates, so spring conditions, when bioconversion efficiency is at minimum and the fish are waking up, favor increased algae growth. Note that this presupposes a Chicago-type climate. I don't figure your dad's seen a 40-degree day in years!

Stay away from algaecides and water "enzymatic clarifiers". The former will foul the water with dead plant material and both will increase the dissolved organic load in the pond, degrading water quality and impairing fish health.

Bob