The common cable ties sold in HD, canadian tire don't have any metal part in it.
Yes it does look funny when you first look at it. But after you corals grow over it and cover most part of the cable ties, you won't notice them any more.
I have used them in my tank over a year without any ill effect that I know of.
Here is a picture showing some of cable ties. I hope the motipora grows quickly and cover the cables.
