Its normal behavior for a YT...mine does it all the time...sometimes I think my tang thinks my 65g tank soley belongs to him and no one else...they are a very dominant fish and often control the whole tank...the one I have is about 3-4"...Just make sure it eats well...give it veggie flake, algae sheets on a algae clip, and mysis shrimp...I soak the mysis and algae sheets in garlic extract...
imo so long as the tang is fat and colorful yellow its ok...the 55g is an alright size tank, just be careful what other fish you put in there. I have the YT and a maroon clown in the 65g and I now want to add another fish, but I am having a hard time because I do not want damsels and I have corals, so really do not want to risk dwarf angels, this all combined with the fact that tangs and maroons are very aggresive...
I'll end up going with a wrasse of some kind probably,
But as far as your YT I wouldn't worry too much...so long as you keep it fat and the water quality is good...
good luck