Drywall compound or mudI was waiting for some attempts first, I assumed this one was so boringly easy, no-one could bring themselves to try!
To be honest I thought it's be guessed immediately! I figured all those lines would give it away.
Ok, well, I'd happily give you a clue but at the moment it's not a convenience for me.
But would that be papering over the cracks? I used to be one of the earliest sources of the chemical industry, though it didn't stop me being called a stinker for it!