I'm not positive but I think mylar is safe to use. I mean, don't they have them ziplock bags made of mylar used for food?
What kind of worm is that?
What about heavy tin foil. Don't know if it would cause any issue but you can research it.
Aquadvisor is a great guide and a good place to start for new fish keepers, but is by no means the authority on what you can and can't do when keeping
I like to stick with natural methods, mopani driftwood, or peat in the filter.