Look like a fungal infection. I would raise the temperature to 85F or so, and add 1 tablespoon/5 gallons of water of salt. Table salt is fine (I've used Norton Table salt without any problems).
Or you could buy medication. I remember using Jungle tablets a long time ago, and they worked as expected. Just read the instructions carefully... and you might want to use a different tank, because that stuff stains the silicone.