I used to have this on my undershirts. It’s 100% anti-perspirant.

If you don’t mind sweating, switch to just deodorant. There’s a difference. Anti-perspirant cakes and stains to keep your sweat from becoming liquid. Deodorant just stops smell, and the sticks usually aren’t pure white.

I use a salt stick deodorant and it’s great IMO. That’s the same thing as the “potassium aluminum rod” sometime else mentioned. Easy to find, never even know it’s there.

Wash your whites in hot water and white vinegar, then usual wash with oxyclean, to get rid of the stains in what you have.