Hhmm.... Possibly did you not teach it it but only have it decline learning the fairly useful grass type move? If not to my question, I have run into glitches like this quite a few times in gen 3 games and especially in gen 5 games. While I may not know what is causing it I can say It is a glitch. To "cure" this monstrosity you'll have to speak to the move tutor. Someday in the future she will eat your heart scales up. Also don't feel so down. You can trade a Luvdisc for a Steelix at Cyllage City's Pokémon Center!
Its name is Thumper.