[2] It is native to western North America, from southwestern British Columbia to northwestern California, with a more isolated population in Idaho.

[2][4] The blue-grey taildropper has a narrow margin surrounding its foot, which is typically light grey or white in colour, and it produces a clear mucus.

[4] The blue-grey taildropper, despite its name, has been described in several colours, including blue, blue-gray, dark gray, brown and black, all observed with opalescent spots.

[8] It is a member of the genus Prophysaon, the species of which can be found in the Pacific Northwest, from California to southern Alaska, and are known for the autotomy of their tail.

[4] Prophysaon coeruleum has an extent of occurrence of 658 km2 (254.1 mi2), however the fragmented nature of its distribution means that its index of area of occupancy is measured at 56 km2 (21.6 mi2) as of December 2015.

[4] This low dispersal ability mixed with their specific habitat needs (moist and cool forest floor litter), means that if this species is extirpated from an area, it may be difficult to recolonize.

[5] It was found in second-growth forests, with mixed-wood (coniferous and deciduous), and at elevations lower than 250 m (820 ft) above sea level.

[5] Observations from the United States, however, are more varied and include both coniferous and mixed-wood forests, and have been found at higher elevations (up to 1,650 m (5,410 ft) above sea level).

Its ability to self-amputate its tail is a behaviour that is triggered in response to disturbances, and likely serves as a defense against predators such as beetles.

[4] During field surveys, this slug has been found both alone or with several individuals near each other, although it is unclear whether groupings occur due to the presence of preferred habitat or if is socially-motivated (eg.

[4] These animals may include vertebrates such as various types of birds, amphibians, and small mammals, as well as invertebrates such as ground beetles and Lancetooth snails.

[5][4] There may be competition between the blue-grey taildropper and other invertebrates, including introduced and invasive gastropods, which may have a negative effect on Prophysaon coeruleum populations.

[4] Other potential threats which may affect the success of the blue-grey taildropper include the development of commercial and residential buildings, as well as transportation and human recreational activities.

[5] The Washington Department of Fish and Wildlife's website lists the threats to this species as including climate change, which would lead to lower quality habitat (eg.

[10][5] NatureServe evaluated the status of Prophysaon coeruleum at the global, national, provincial and state levels.

[4] Of the various threats COSEWIC lists as potentially affecting the blue-grey taildropper, "natural system modifications", "invasive, non-native species", and "climate change and severe weather" are the three which have been estimated to have the most impact.

[10][14] The Washington Department of Fish and Wildlife's website lists the threats to the blue-grey taildropper (see above), as well as the conservation actions required to ensure their survival.

[10] These include the protection and monitoring of sites where they may be found, as well as more research into this species' habitat needs, range, and taxonomy.

[10] Idaho Fish and Game lists Prophysaon coeruleum's state conservation rank as being S1Q, meaning that they are "Critically Imperiled" (S1), but that there is some "uncertainty about taxonomic status" (Q).

[16] They are also listed as a "Species of Greatest Conservation Need Tier 1" in the Idaho State Wildlife Action Plan.