Should I clean out the yellow greasy stuff that came on my new crank/BB?
 

Or is it good stuff? I'll be applying more grease either way.

New? IDK .. Campagnolo's traditional grease was that color 40 years ago, lithium type I think..

I Cannot help you with your problem solving outsourcing decisions ..

I remove the grease because it use for packing and shipping only and replace with whatever grease you use to pack your bearings .

Stuff that comes INSIDE bikes is best left alone until time to relube. Stuff on the OUTSIDE doesn't serve any purpose and probably should be wiped off with a rag.

If it's along the tooth line, it's most likely transferred chain lube that dried over time, and it's your call because there'll be more transfer anyway.

That greasy stuff is called grease. It's there for a reason, although it may have been applied in such a manner that there is excess on the external surfaces of your bike. As was mentioned above, the stuff on the outside is just excess, and can be safely wiped off, whereas the stuff that is on the internal threads or bearings serves a purpose. You should grease those things when building up a bike, but in some cases, it may have already been done for you.

Sorry, I suppose I should have given more context. I just got a 105 5800 groupset and it's on the parts fresh out of the box. Looks like it's just inside the BB and on the part of the chainset that goes inside the BB, so it's probably something worth keeping on. Just wanted to make sure it wasn't some weird thing to preserve the parts in storage or something. Thanks guys.

There is never preservative on bike parts. They are made to sell, not store.