Does anyone know how to fix a leak you can't find!

we have a leak in our house, right in someone’s bedroom, we live in a massive warehouse so it’s exposed ceiling. The roof above (nyc) has black spongy stuff on it and finding the leak is almost impossible, are there some tricks we can do to stop it dripping into her room?
It runs down a pipe then drips onto her floor, but there is a lot of it.

Any ideas?!