The lichen was formally described as a new species in 2021 by lichenologists Pieter van den Boom and Pablo Alvarado.

The type specimen was collected south of the village of Zijtaart, where it was found growing on a medium-sized ash tree along the side of a road.

Several morphological features of the thallus distinguish Catillaria flexuosa–it is dark green and relatively thick (up to 0.4 mm), continuous to weakly rimose, areolate, with knobby granules, and it can sometimes become somewhat squamulose.

They are typically flat with a small but conspicuous proper margin, and often have a flexuous form–bending in alternate directions.

[1] Catillaria flexuosa is a corticolous lichen, and grows on the bark of Fraxinus excelsior trees.