My 5 week old chick has a case of rotten butt

I have 9 chicks 5 from a breeder that is doing great and very affectionate, and 4 that I got before the 5 from the breeder, they was gotten at a feed store.  Well my rhode island red who is the smallest about a few days after getting started looking like it's rear end is white and all around it looks rotten.  It won't grow a tail either.  It is eating, drinking and acting like normal but it looks gross.  Any ideas of what it is and what to do for her?  Appreciate it in advance.

Thanks great ideas.  I did indeed soak her in warm water to losen the gross stuff off and cleaned her up.  When I cleaned off all the stuff her skin was bald but no cuts or lesions, nice and pink.  Her vent was was normal looking and clean afterwards but bald from her tail to half way up her belly, or breast or whatever you want to call it.  LOL  I put antibiotic cream on her and in the morning I will see if she is doing better.  I sure hope so.  I have never seen stuff get caked on and cause balding and really look so bad.  Nothing looked infected, but the anti biotic cream will at least keep the stuff from sticking and get her some comfort in case she is raw.  No pun intended.  So hopefully all things will get better.  Like I said, she is eating and drinking and acting normal as the others but the rotten look just took me back.  Great ideas everyone, awesome to have back-up.

Ok she is older now and she is growing fast still not as big as big as the others but she seems healthy and happy.  She still has the problem but not as bad.  Her older feathers has developed weirdly.  On her back torward her rear it is shaped like a upside down U.  Odd I know but she is healthy drinking well and eating well her feathers are grown in nicely but weird shaped.  I don't know what to think.