My black neon tetra keep rubbing against my tank...me newbie..how to handle??

hi all..

I need help.. 1st time keeping fishes..and i spotted one of my black neon look different from the rest and it behaving differently..

The top portion , above a silver lining across its body, is red and it keep rubbing itself against the side / bottom of the tank..

It is eating normally..but i am still concern...

So far only one black neon is behaving like that.

I have cardinal, black neon, cherry barb and albino glowlight in my tank..

Please help..