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Clean them off with 99% Isopropyl Alcohol.
Do not touch the Thermal Pads with the 99% Isopropyl Alcohol.
That part of the Water Block is not going to hurt anything.

 
Moved to the 20 Series section.

I believe that's normal.
It's very rear to meat waterblocks today perfectly build. 
Every brand I saw have traces of production on places where waterblock not touch GPU or processors.
Brands don't even try to polish better surface except on contact spots. Many of them have circles of machine from production process or similar traces.
 
I must check Watercool blocks, for them I'm not sure, but EKWB, Bitspower and others for sure have.
Only spots in direct contact with hardware are polished better and that's sad how much they cost.
It's not only waterblocks same is with acrylic or acetal parts example pump tops and etc... channels for water are not polished pefectly and have micro obstacles.
I believe something like that would demand more work and increase production price and that's reason why they left after CNC milling finish without polishing better except spots who touch memory, GPU, CPU or PCH, everything else inside have traces of production.

Looks completely inconsequential to me.

That is how my two block looked too.
Flow super sucks for that block FYI. Unless you are doing SLI or something, you might want to use the pretty good cooling that came with the card.

Those parts of the block don't contact anything. Even if there were aggressive machining marks there it wouldn't matter. 
 
The flow or blemishes won't be the problem you have with that block. Post back when you get some temps. 
 
Or, send that back and buy a EK Supremacy GPU block and run that with the KPE copper heatsinks installed. $60 and will perform better than that $300 block.