Aqua Lab Rat is correct. Just wipe the small amount of brownish residue with a cloth. This can occur after a complete hot tub purge. A good flush product has to penetrate the biofilm outer layer to gain entry so it can begin dispersing and lifting the gunk from the colony. This is why all flush products want the user to run the pumps as the flow of water helps to carry this detritus material into the hot tub proper. Sometimes, small amounts of the sticky gooey stuff is almost broken free but is still hanging on. A few days after running the hot tub it breaks loose and this is what you are noticing. No big deal. Just wipe it up. Now, for Certified 106. You did good. Start enjoying your hot tub. The water chemistry testing you are doing is great to learn for new owners. However, now is the time to back off a little and Follow the basics. Once you removed most of the biofilm, as you did, your water maintenance will be very easy. Do check pH, alkalinity and cyanuric acid periodically, but don’t be paranoid about it. The important thing to check on a regular basis is the amount of available sanitizer you have. One of the best investments you made was getting the TF test kit. After a few weeks you’ll get to know your tub’s requirements. It will become second nature for you. Just purge and drain the hot tub at least twice a year, adjust water to accepted chemical parameters, maintain adequate sanitizer at all times and you will be enjoying crystal clear healthy water. Enjoy!