Helocarpon erraticum is a species of crustose lichen in the family Ectolechiaceae. It grows mainly on siliceous rock and is a widely distributed species.
The lichen was originally described in 1861 by German lichenologist Gustav Wilhelm Körber as Lecidea erratica. It was later treated in a broad sense within Micarea, and in 2009 Richard C. Harris and James Lendemer erected the genus Leimonis for it, emphasizing its saxicolous habit and its comparatively well-developed thallus and .
A five-locus phylogeny published in 2026 recovered Helocarpon as a strongly supported clade sister to Micarea and showed that Leimonis (based on erratica) falls within Helocarpon as circumscribed there. That revision reduced Leimonis to synonymy and made the new combination Helocarpon erraticum.
In the 2026 revision, H. erraticum was described as forming a rather thick, olive-green to grey-green crust that cracks into small angular patches (âÂÂ). Its fruiting bodies (apothecia) are numerous, very small (to about 0.4 mm across), and dark brown to blackish. The spores are narrow and colourless, and are usually but may have a single septum (1-septate).
Helocarpon erraticum is mostly saxicolous and occurs chiefly on siliceous rocks, where it can behave as an early coloniser of exposed mineral surfaces. The 2026 revision characterised it as widely distributed.