Some time ago, the fat ball had plotted against White Two, enticing him to go to the world of the black lotus so that it could use its exceptional sealing techniques to seal him inside—sealing techniques had always been its strong point, and also the skill it was most proud of.
After getting sealed, Senior White Two had not gone to the Netherworld again to fight the fat ball... part of why this happened was that he found some fun things to do in the main world.
Today, it was mainly because Song Shuhang had appeared in the Netherworld Realm while using his exoskeleton armor, which he had been able to sense, that he became curious and decided to leave the world of the black lotus.
The liquid metal ball had designed its lair to have an airtight defense, so even someone like White Two would have a very difficult time getting in.