Translation:If I had a nickname, I would be happy.
You probably mean maljuna, the opposite of young. Malnova is old as in the opposite of new.
For example, if someone bought a kitten a while back and now buys an older adult cat, then the adult cat would be la maljuna nova kato, while the kitten would be la juna malnova kat(id)o.
This is not that kind of nickname. (that would be kromnomo, an "instead name") Kaŝnomo comes from kaŝi (to hide) + nomo (name), so it's a name you use so you don't have to use your real name.
It's also a synonym of the word pseŭdonimo, although that is more specifically "kaŝnomo de verkisto aŭ artisto", according to PIV.