Use of green tea to flavor beer

I've heard of this being done a couple different ways but more or less the same set of variables as with adding coffee to beer. Adding the tea to secondary for a few days to effectively cold-steep there is a decent option and should work out fine. Weighing it down in hop bag may not be a bad idea but probably isn't super necessary.

The alcohol already present in the beer during secondary should be enough to take care of anything that might be introduced with the tea but you could also consider rinsing it with near-boiling water for maybe 10 seconds before adding it to try to quickly kill anything that might be on the leaves. Many people that drink loose-leaf tea do this anyway to remove any pesticides that might be on the tea and to "open up" the tea leaves before the full steeping.

Another option is adding pre-made tea (hot- or cold-brewed) directly to the fermenter which has the benefit of being pretty well-tunable to the amount of tea flavor you want. Take a sample of your beer (or one from a prior batch) and figure out the proportion of tea to beer that gives you the flavor you want.

You could also make a tincture by soaking the tea in vodka for a few weeks so that the flavor will be extracted by the alcohol and add that. This is generally done with herbs but I believe it should work for tea as well. Even if you were just soaking the tea leaves in vodka temporarily to kill bugs I think you'd still want to add the vodka and all to the fermenter to keep anything that was extracted by the alcohol.

Ultimately, you have several options and they should all be pretty safe and repeatable so long as you measure things out carefully.