Got this rock from existing tank at my lfs. Should I be worried? I think I saw something sticking out of one that looked kinda like a feather duster. I’ve had no problem with mucus coming from there yet.
It's hard to judge from the photos - I don't think I see what you are referring to.
Generally speaking, without knowing anything about your tank or your LFS, I don't think you should be overly worried - cultured live rock provides a lot of benefits to your tank which typically outweigh the negatives from having undesired biological hitch-hikers that inevitably come along for the ride.