First of all how long has the tank been running with betta in it. Is it cycled. New tanks that are cycling can get brown algae in them. What are water perimiters. What type of filter or do you even have one for the tank. The answers to these questions would help. It looks like you have no filter in your sig, if not are you changing 100% of water weekly, if not you have to, your betta will eventually die. Need more info from you to help answer your question.