The lichen has a squamulose (scaly) thallus, with a range of bright yellow to greenish-yellow and brownish-orange colours in its soredia (powdery propagules) and apothecia (fruiting bodies), respectively.

The apothecia have dark brownish-orange discs, surrounded by slightly paler yellow margins, with the spore-bearing asci containing typically eight brownish-golden ascospores.

[3] In 2013, the taxon Caloplaca ferdinandmuelleri was formally transferred to the newly proposed genus Filsoniana, as part of a molecular phylogenetics-informed restructuring of the family Teloschistaceae.

[4] Gintaras Kantvilas (who prefers to maintain this taxon in a broadly defined genus Caloplaca) suggests that it is likely a sorediate counterpart to Filsoniana rexfilsonii.

[5][6] Filsoniana ferdinandmuelleri is characterised by its squamulose thallus, which has a vibrant colour palette ranging from bright yellow to greenish-yellow soredia (powdery vegetative propagules) and brownish-orange apothecia (fruiting bodies).

They are scattered or weakly aggregated, featuring dark brownish-orange discs that lack a powdery coating (pruina) and are surrounded by slightly paler yellowish margins.