Columnaris isn't a fungus it's a bacteria.
I would be very careful using anything that contains Malachite Green (also a great anti-fungal) on a scaleless fish unless buffered with Triple sulfa. You also have to be aware of the Ph and hardness when using it. On the other hand and why I recommended Methylene blue is it is very mild, fish tolerate relatively high doses so accuracy isn't important and is one of the best anti-fungals around. It is hard on BB because it also posses gram-positive antibacterial properties. It will kill fungus (saprolegnia et al) and due to it's blood oxygen enhancing properties aid in the healing of tissue among it's other uses. It can be used as a thirty minute bath dosed at one teaspoon per 5 gallons of water. It may also be used poured directly on the effected area and the fish may be put directly back into the display tank. The title of the thread is "Best fungal treatment" considering the species and the issue this is it.