A couple of weeks ago I planted some valencia orange seeds, which are now starting to germinate. Today I noticed a seedling that is all white. Could this be a variegated seedling or is it albino? If it's an albino it will probably die soon...

Well, I know one thing for sure - you can't get variegated citrus from seed.

Looks albino. Probably won't make it. I'm pretty sure you can get variegated from seed as long as a sexual cross takes place.

Crossing has nothing to do with variegation.
Variegated plants are chimeras.
As a seedling is made from one cell we might no be able to get a variegated plant out of a seed.
But we can imagine a mutation happening in one cell of the first meristem(s) leading to a variegated seedling. In fact few people have claimed that they obtain variegated seedlings.