You should never use algae killing chemicals in fish tanks. They can often kill livestock.
It appears to be BBA. Without knowing any details about your tank BBA is often caused by leaving lights on too long. While you have an algae issue you should only run your lights 6 hours a day, use a timer if needed. If the log is removable it's best to take it out of the tank and pour hydrogen peroxide over it. Let it sit 20 minutes then put it back in the tank. If it's on plants I can tell you how to spot treat them. But you can't do this now if you have an algaecide in the water.