More than anyone might want on this; a few more photos -
https://www.flickr.com/photos/wildhastings/329405361/
https://www.flickr.com/photos/149980226@N06/53134210924/
https://www.flickr.com/photos/149980226@N06/52131159298/
https://www.flickr.com/photos/72842252@N04/30530281196/
see caterpillar photos in particular -
https://www.wildlifeinsight.com/british-moths/brussels-lace-cleorodes-lichenaria/
from Wikipedia -
The Lithosiini are a tribe of lichen moths in the family Erebidae.
Lithosiini (Arctiinae, Noctuoidea) larvae are unusual in feeding on algae and lichens (hence "lichen moths")
(an aside - “Adults of some Erebidae (also Noctuoidea) pierce fruit to suck out juices (leading them to be called "fruit-piercing moths"), and those of Calyptra can also pierce mammalian skin to suck out blood (hence "vampire moths”)"
In “Lichens” by Oliver Gilbert (2000) (great book, difficult now to find or afford; some years ago was able to get quite a good print on demand copy from HarperCollins) - “The scalloped hazel (Odontopera bidentata), for example, has a ‘stick’ caterpillar that feeds on a range of trees. When nearing maturity the larva has the ability to turn either a mottled bluish-green, mimicking a lichen-covered twig, or dark purple, resembling a bare one, depending on its surroundings. In addition, the adult moth, which spends the day resting on trees or fences, is well camouflaged against a lichen-covered background. The Brussels lace (Cleorodes lichenaria) is even more closely adapted to a lichen-dominated environment, with a greenish-grey caterpillar that feeds on lichen and resembles a lichen in colour, form and attitude so precisely that it is almost impossible to detect, while the adult moth displays an elegant mimicry of a lichen-covered surface.”