You could try some Dawn DW detergent .Rub it in well
and let it sit for awhile. Then scrub by hand.
Or a shampoo might work.

You can also mix up 1 part Dawn to 2 parts hydrogen peroxide. Put that on the stain. I have read where some people add some baking soda to the stain at this point and rub it all together. Then let that sit for an hour or so--then wash.

Wisk detergent might get it out also. For years it was the one for "Ring around the collar" and now it is supposed to be even better on body oils/dirt. It rates at the top in Consumer Reports tests.

Are you sure it is just sweat? it almost looks like sunscreen or some other similar stuff. If it is a chemical it may never come out.

I was thinking sunscreen, too. My hubby's got some stains on his golf shirt collars that will not come out, and they look like dirt and only in the back. I think they're from sunscreen, in spite of me never using an oxygen bleach or detergent with oxygen bleach in it.

try Dawn as someone else said.