+1 on Bryopsis ID. And yes, the various species can look noticeably different.
According to AlgaeBase, there are currently 55 accepted species in the genus
Bryopsis, plus another 9 accepted species in the closely-related genus
Pseudobryopsis.
The common denominator across all
Bryopsis species is the presence of erect feather-like uni-axial fronds. The fronds can vary in size/length (from a fraction of an centimeter, to as long as 40 centimeters). For certain species, the feathery structure is so fine that from afar, the alga looks more filamentous (like hair algae), rather than feathery.
The other common denominator is that almost all species are very efficient in propagation, with multiple patterns of reproduction. They are tolerant of the widest temperature range, can survive extreme swings in salinity (even in brackish water), easily adjusts to different flow patterns (from turbulent rocky surf areas, to very placid protected lagoons) and to different levels of lighting (from perpetually unlit areas shaded by mangrove trees, to dark deeper water more than 5 meters deep, to the brightest shallow lagoons under direct sunlight all day).
This adaptability makes
Bryopsis a very formidable opponent. One unchecked spot can quickly spread and become a tank-wide infestation, thereby becoming a major pain for us aquarists
HTH,
Phil