Thallus superficial, grey green to whitish, supporting abundant coralloid isidia, usually green, grading to pale ochre-orange where well lit, with a glossy surface, forming dense mounds away from the apothecia, more discrete near apothecia, 20–50 μm diam., with frequent constrictions and branching, with a compact clear cortex several cells thick surrounding up to four algal filaments. Apothecia 0.5–1.5 mm diam., sessile on the thallus (not when on isidia), distinctly constricted at the base, disc yellow-orange with pale creamy-yellow, often flexuose margins; hymenium 80–110 μm tall; paraphyses ca 1.5 μm diam., the apical cells to 4 μm diam. Ascus without an amyloid ring around the pore. Ascospores 9–11 × 2.5–3.0 μm. Pycnidia present but no data available.
Very rarely fertile and long overlooked as the similar looking but much rarer Porina rosei. True P. rosei is also often sterile, but is darker orange with matt, softer-looking isidial mounds with narrower isidia, up to 27 μm diam., and a thinner clear cortex of a single layer of irregular rounded cells surrounding a single algal filament. C. nimisii is very similar to the New Zealand species C. fruticulosum (Ludwig 2014), except based on limited fertile material available, the hymenium is taller and the isidia appear more densely packed in the European species. It does not resemble any of the other varied tropical and austral isidiate Coenogonium species; see Aptroot & Cáceres (2018).
Enterographa brezhonega is occasionally parasitic on C. nimisii and rarely on C. luteum, and could be easily mistaken as myxomycete fruit-bodies or blobs of Lepraria; however, if looked at closely the convoluted white lirellae are highly distinctive.