Do you necessarily need the 上 in this sentence?
Yes, shang is 'on'
Is 一些 not needed here?
That would imply that there are at least a few things on their face.
Can't we say 你的脸上有一件事情?
事情 is not a material "thing", it's rather means a "matter", so it's inappropriate to use it in such context
Thank you very much for your explanation!
Why does 东西 not have a measure word here?
Would 你的脸上有什么 be correct?
Could any native speaker please confirm: as of today duo is pronouncing 東西 as dong1xi1, but shouldn't it be dong1xi (xi neutral) here?
Both are actually correct. It does depend on the region and dialect, but it's much more common to hear dong1xi (xi neutral).
why is 你的脸上有个东西 right?