I use lichen a fair amount in some pola buildings, ground shrubs, planter box fill.
It has a look, and, not a great one, imho, unless augmented with flower material, or, finer shrub and tree stuff.
Its cheap, common in ho scenery supplies.
I use it in my pola stuff because its cheap and replaceable.
It dries out, but, if untouched, seems to more or less work. A spritz will rehydrate it. Seems the color fades with dryness and returns with moisture.
It seems too, I tried the glycerine thing decades ago, and, never kept it up. Messy, and interferes with adherence as i recall.
I also use lichen in my dry denver n scale layout, stuffed into utah desert mountain crevices, bottoms of arroyos, etc., I find that it stays, and, should the basement humidity rise, as it does on occasion, the lichen softens. Otherwise its dry, but seems to work anyway.
I havent found a solution to keep it soft, other than high humidity.