You’re right; it’s about pee.

Apparently cicadas send a stream of pee instead of droplets like most insects. Scientists studied them since a stream of pee should use more energy at their size and they are studying why/how they still use that method.

https://phys.org/news/2024-03-cicadas-unique-urination-fluid-dynamics.html

I mean, I first thought blood, but I think you’re right