Help with (Rubber?) Lichen

Probably has been well soaked/treated with glycerin. Some plain old white glue should do the trick just fine, as always when in doubt, test it on some scrap.

Lichen looks like...lichen. Why not invest in some of the better foliage products. Even  a visit to the local Michael's or AC Moore floral department will get you more realistic products...many at much better prices than the "Hobby" based stuff.

Or scrape some off the asphalt out front. Nothing looks more realistic than the real thing.

But if its rubberised then white glue or CA would do it. Only issue with CA is it may eat the product.

I used to use that stuff to attach to vehicles. And yes, white glue works fine. But its not very realistic. I don't even buy it any more.