Canteen guys, I need some help, please

[Kev308]

I put my canteen away after hunting season and when I dug it out today I saw this crud in the neck. I was stored dry and the cap was off. What caused this? And more importantly how do I get rid of it?
Thanks for your help!
Kevin

 

[SOLANCO]

Looks like aluminum. If so try boiling water with vinegar. If stubborn, try a paste of vinegar, salt and flour.

 

[sturmkatze]

German, right? Do like @SOLANCO says.

 

[Salty Cracker]

I'd stick my finger in it an rub it Round and refill

 

[Omahkapi'si]

Calcium maybe from hard water?

 

[charlesk]

If is wasn't completely dry on the inside when stored, could be its cause is hard water, mineral residue left while evaporation or after caused the corrosion. I've seen this on others.