The fruit body appears on the lower side of dead wood and, from a distance, looks like a wash of colour on the wood. The colour may be blue-grey to yellowish green. A closer look shows the structure to be a somewhat fibrous mat (much like dense, compressed cobweb or flattened fairy floss). This mat is attached fairly loosely to the wood. Fruit bodies of Amaurodon species are bluish when fresh but dry to yellow-green. In the field, depending on conditions, you may see only bluish shades, only yellow-green or some mix or intermediate.
Spore print greyish blue.
Amaurodon aquicoeruleus is a relatively recently described species, the first published description having appeared in 2001, based on material collected in Western Australia. Amaurodon spores are initially blue to violet in an aqueous solution of potassium hydroxide (but the colour may fade and change to some shade of brown after many minutes). In general, they are colourless or yellowish in water but the spores of Amaurodon aquicoeruleus are distinctly blue in water and those of Amaurodon angulisporus (a new species reported from west Africa in 2011) become pale blue after some minutes in water.
If I see a loose yellowish-green mat on wood I usually think of the genus Amaurodon but I do check it under the microscope to be sure. A few times I have come across such mats which I have found to be the asexual stages of some fungus or other, with colourless, tightly coiled asexual spores. For simplicity, given my ignorance, I file such collections under Helicomyces (where they are at least easily findable) but I do know that there are more 'coiled' genera than just Helicomyces.
Amaurodon aquicoeruleus is listed in the following regions: