3 week old chick keeps pecking under his/her wing

[steveclank]

Chick is acting "normal", eating, drinking, pooping etc...but keeps pecking under her/his wing. I'm a complete newbie with chickens, they all have been great so far. No other looks like this, but the others(total of 9) sometimes rub their beaks under there wing. Is it normal behavior? The pics below don't look normal. Any help would be appreciated. Pic was taken last night, chickens acting "normal" this morning.

 

[CMV]

What's the goo under there? Is there a wound?

 

[steveclank]

I didn't notice any blood last night. I put them outside for the first time the day before. Not sure if it's something from outside, or could it be poo? I keep the brooder as clean as I can get.

 

[CMV]

That is not area where you are likely to see poop. Clean it up and see what is going on in there.

 

[steveclank]

What is the best way to clean it?