White phosphorus in general is not a war crime.

General document for covering this is the UN Convention on Certain Conventional Weapons, in this particular case Protocol III, Article I §1(b)(i).

Munitions which may have incidental incendiary effects, such as illuminants, tracers, smoke or signalling systems

Of which White Phosphorus is typically within that category. It is incendiary and can randomly burn people, homes, and fields but the intended use is usually not to set fire but to provide cover, illumination, tracing, or signalling. There just happens to be a burning side effect that the UN usually shrugs off.

How it's used however can become a war crime. Article II of the same Protocol § 2 and 3

It is prohibited in all circumstances to make any military objective located within a concentration of civilians the object of attack by air-delivered incendiary weapons.

and

It is further prohibited to make any military objective located within a concentration of civilians the object of attack by means of incendiary weapons other than air-delivered incendiary weapons, except … and all feasible precautions are taken with a view to limiting the incendiary effects to the military objective…

So basically, you cannot deliver via aircraft, which video indicates that these were ground launched 155mm white phosphorus artillery projectiles, so check on (2), BUT it's debatable that Israel followed (3) and took all precautions to minimize civilians getting in the way. But I mean, does anyone really believe the UN holds anyone to any kind of scrutiny?

So… I hope that answers your question.