The "Mars" guy was J'onn J'onzz, the Martian Manhunter (whom I love in JLU). And maybe I have my Clark tinted glasses on, but his decision to go to Seattle over helping Lex didn't(IMO) have anything to do with his feelings towards Lex. There were people being literally ripped apart and Clark had to do something. Lex goes missing every other episode, and Clark has been the one to save him everytime. In the case of a missing person: something that the police can presumably deal with; and people being eviscerated by an alien: something that the police have no clue about nor the means to stop (especially if the eviscerated cop is anything to go by), then yes, Clark made the right decision.