I had a small one do this about 1 week ago, near the same spot in the tank. This time, there are 3 "plants", all are much larger, doing it.
The entire algae changes at the same rate, over the entire surface of the entire algae. It's gone from perfectly healthy looking to this in less than 12 hours. In 12 more hours, it will be a completely white skeleton. Meanwhile, other Halemida in the tank are unchanged.
I'm going to take a look at some of it under the microscope and I'll let you know if I see anything. I'm sure it will be interesting, but I will have no idea of what I'm looking at!