Yes that looks exactly like it. Been battling the hydromedusas on my rocks, didn't realize they could morph into this huge jellyfish.
These will go away eventually..?
The Medusa form is a stage of their life. They go between the Medusa, motile form, and the stationary, polyp form. It will not go away as long a they are reproducing in your tank
Just added 2 mithrax, 1 emerald and 2 sally lightfoots to my cuc, think they will help control the population..? The snails & urchin don't make a dent as the hydroids are only in unreachable dark caves and crevices. Shrimp don't touch them either.