Civil-Environmental engineer here, looks like your septic system or grey water discharge is filtering thru the ground and entering the brook, causing the foam you see in the pond where the brook enters the pond.
That’s a pretty common problem with rural housing. The foam is caused by detergents, so a malfunctioning grey water (laundry) system is probably the culprit. Grey water systems are not regulated as tightly as septic systems and therefore sometimes are not properly sited, designed, or installed. If the home does not have a grey water system then, unfortunately it’s the septic.
