This was hang from and up under cast iron drains. What is it and what caused it?
not sure about white stuff.Is it hard or soft ?looks like water logged expanding foam.
pipe looks like galvanized drain pipe to me!
Looks like crystallized drain cleaner, like Drano.
Could just be real hard water mineral deposits from the obvious leak, but if you add drain cleaner to hard water and let it slowly leak, you’ll have stalagmites.
This drain pipe is galvanized which connects to cast iron just below this pic. The white stuff is hard and there was evidence of a past leak.
Looks like the Mercer Caverns to me…LOL
I agree with Mr. Duffy! I have seen this before and always tell my clients that if the home has cast iron plumbing pipes, not to use draino or liquid plumber. Those things eat cast iron pipes up and then leak in the crawl space.
Looks like your inspecting the Flintstones home! 
Was this a house or a cave?
Stlagtights and stlagmites. This is probably a leak and it is heavy water (not as in nuclear: but un-softened). This will happen with other drain materials too such as PVC or ABS. Was this a shower pan? Looked like it.