By "xylariacous" is meant members of fungus family Xylariaceae, which is known so little by the general public that it doesn't really have a good English name. Sometimes members of the family are called "wood decayers," but members of other fungus families also decay wood. This is one instance when it's probably best just to use the Latin name, even though members of the family are quite common. You'd say "It's a member of the Xylariaceae, pronouncing the word as if it began with a Z.
The black-and-white little fungus at the right (only 0.4 inch or 10 mm high) is a member of the genus Xylaria, growing on the wood of a log fallen in the woods. Most people would never notice this little being unless they were specifically looking for small things, poking around on fallen logs.
At the left you see another member of this family. I think it's a member of the genus Hypoxylon, but I'm not sure. It's like a blistering skin growing on the lower surface of a dead twig. The image is magnified about four times. This fungus is a parasite on oaks and other hardwood trees, but usually it only attacks trees that already are very ill or almost dead because of other reasons.
The crustlike body of such fungi go by the special name of stroma. If you look at the stroma very closely with a handlens, you'll see tiny (less than 1/32-inch, 0.8 mm), black, slightly raised dots. These dots are the tips of small cavities called perithecia (sing., perithecium). Perithecia produce ascospores which are discharged to the air. These ascospores then germinate to form new stroma.
All members of the Xylariaceae should be considered inedible.