Parmotrema perlatum aka Stone Flower.
This lichen is used as a flavouring in Asian cooking. Having examined it through a lens it looks like Parmotrema perlatum. It has holes, is pearl grey on upper surface with soralia on the margins. I have not tried cooking with it yet but it has a pleasant smell.
Those below found in Iris Brickfield are Physcia caesia and Xanthoria parietina. They make an interesting colour contrast.
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