If this "yellowish-brown stuff" is yellow/mustard algae, then instead of a minimum FC level of 7.5% of the CYA level (which keeps away green algae), it takes a minimum FC level of 15% of the CYA level to keep this algae away. At a CYA of 80 ppm that's an FC level of 12 ppm to keep this algae away from being reintroduced.

Also, the shock level for yellow/mustard algae is higher than for green algae, but with such a high CYA level it's probably impractical. You should also make sure that the yellowish-brown stuff isn't dirt or pollen though it does sound like algae.

Richard