Fluctus is a cloud formation that requires quite specific circumstances to occur. The physical phenomenon itself (Kelvin-Helmholtz instability) is quite common, occuring at the border of two layers of fluids moving in different directions and/or speeds. The most common instance would be the waves you see on water.
But to be visible in a cloud like this, there not only needs to be a cloud at that same place, but it has to also occur perpendicularly to the observer so you can see it clearly from the side like this. Because of these three factors rarely happening all at once, as well as the short lifespan of the formation (the waves usually crash within minutes), it is widely considered a rarity.
Safe to say, I was very stoked when I saw one lining up for this shot.
