Knowledgebase
Moss, Lichen, Extraterrestrial Invasion???? #749466
Asked May 12, 2021, 12:04 PM EDT
Montgomery County Maryland
Expert Response
It's hard to concretely identify while dried and flaky, but this appears to be a type of Liverwort. They are cousins to moss and occur in similar environments - often shaded and damp. They don't have true roots, but can have tiny root-like structures that help them hold onto their substrate of choice. If plants are reappearing after removal, it's not because of left-behind roots in this case, but rather than the environment is well-suited to their growth and they are returning as a new generations from spores.
There are dozens of species found in MD; two commonly-encountered ones are shown in image galleries here, which you can compare to yours (when green); both are native:
https://www.marylandbiodiversity.com/view/8264
https://www.marylandbiodiversity.com/view/8262
They shouldn't harm anything if left alone, but if undesirable due to creating a trip or slip hazard, keep physically removing them and try to improve conditions so the area is less suitable for growth. Treatments with a registered herbicide product that is labeled for liverwort control (and likely also moss control) might clear-out existing populations but won't prevent new colonies from establishing later, unless it has a long residual. If the brick pavers are not near any highly-flammable material, you could try a flame weeder tool to see if it sterilizes the gaps between bricks. This will be a temporary measure, though, if the area stays moist enough that spores can re-colonize the area. (Is there a sprinkler system or leaking hose/downspout that frequently dampens the bricks?)
Aside from moss, which is treated like liverwort, the other possible identity of this residue is a cyanobacteria called Nostoc. They are fairly easy to differentiate from mosses and liverworts when living and green, as Nostoc has a jelly- or seaweed-like consistency, though they too darken and crisp when dry and re-animate or quickly re-colonize an area from spores after removal. Clemson has some more information on this organism: https://hgic.clemson.edu/factsheet/nostoc/
https://www.marylandbiodiversity.com/view/20912
Miri
You're welcome.
Horticultural-strength vinegar (acetic acid) may work, just use extreme caution in applying it as it is not only caustic to human tissues but will damage nearby plants as well. (Note: using household vinegar won't be effective, of which you may already be aware.)
Miri