White phosphorus is pyrophoric, which means it will self ignite when exposed to oxygen and burn. Which is why it is stored in water. But it does not explode and certainly does not detonate as shown at the end of the episode.
The team finds one suspect trespassing on private property. He then goes into a spiel about how they have no evidence to hold him. In the next scene, the team talks about having no evidence and not being able to hold him, yet they just found him trespassing on private property! They may not be able to hold him for the arson, but they can hold him for trespassing.