It is the rehaut for the crystal. It comes with the crystal and they are readily available and it should be the silver one for your watch. Cheers
Yes. The rehaut is the ring between the dial and the edge of the case. In omega parts speak, 'white' means silver. That's the one you want.
That's in general jeweller's speak, at least I know it like this from the strap buckles. If they are not made of silver or gold, they are not called silver or gold, but white or yellow, to avoid any potential misunderstandings.
Rehaut is actually a misnomer regarding the Speedmaster. I've never seen a watchmaker called it a rehaut. It's just a tension ring for the acrylic crystal. It can be called a rehaut if it's used for elevating the crystal above the hands/dial. Most acrylic crystals are already curved and have no need for a rehaut. Without the tension ring, the crystal would sit just fine in it's place. Yours looks like a white/silver one already. Any problem with it? Is that a 2998-1 or 2998-2? More pictures please.
In this case it's a tension ring, because it does add tension to make the crystal fit and stay in place properly. Not all crystals that look like this and have a metal ring, have a proper tension ring. Some metal rings are called a "reflector ring" that adds no tension to the crystal fit. And yes, in trade speak anything silver is "white" and anything yellow gold is "yellow". Tension rings, crowns, etc. all use this, and certainly not specific to Omega. Cheers, Al