Grease isnt bad for them, its just slow. To clean them soak them in a solvent (wd40, gasoline, etc.) for 5-10 minutes. I like to put them in a small bottle and shake them up in it, works faster/better. Then rinse the solvent off with water and dry them immediately with a hair dryer or air compresser so they dont rust. Then put a few drops of that speed cream or another lubricant, I like to use tri-flow with the teflon in it. Then wrap them in a paper towel to dry off any extra lubricant and slap them back onto your wheel. Brand spankin used. Good as new.
And yeah dont ride through water, mud or dirt, but thats common sense.