There are multiple things that can cause brown patches on houseplants. It could be overwatering, rot or a fungus. I’d check your watering schedule, as all three of these can occur when the plant is overwatered. Is it winter where you live? If that’s the case then you should be careful not to overwater, especially if the weather gets dark and gloomy.
…I forgot for a minute there that “mother-in-law’s-tongue” is a type of plant.