What kind of sea hare do you have? Any pics of it?
The mass looks like an egg mass, but I'm not familiar with a sea hare species that lays their eggs on the glass at the moment (I'm not super familiar with sea hares yet though).
Looks like eggs to me also. I am no where near as knowledgeable as @ISpeakForTheSeas so take what I say with a grain of salt. But the zigzag pattern of the mass does look like some invert egg laying I have seen.