They automatically attach a movement debuff of -60% to you, but they shouldn't damage you too much (especially if frost, with barrier up), which is why you should ignore the ones just walking by themselves.
You said you have the extra cloth trained, I got it from a book in Heroic Utgarde Keep, and went straight to Icecrown to farm. 20 minutes = 6 stacks. Anywhere from 2-10 per group, upwards of 4 per mob.
Land on them (or near them), root and kill like a pro, loot, mount and fly to the next batch, rinse and repeat. They have a quick respawn, so you'll never run out.
You come out stuffed with frostweave cloth. You get a lot of money too, stuff to disenchant/mail to a disenchant character, etc.
I never did these before, at the right time. Just recently, I went back and soloed some for the achievements. You get a load of cloth.
Its way better than farming it on that tailor character, just outside.
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