I would say that Palythoa spp. and Protopalythoa spp. could have sand or similar substrate embedded in them. I have found both of them in the wild without any grain of sand or similar substrate in their tissue simply because there was no sand around where they were found. On the upper side of my tank the Palythoas and Protopalythoas don't have a grain of sand in them.
If there is sand or similar substrate in their tissue they are not Zoanthus.
The skirts can vary, yes, and shouldn't be dependable for proper identification.
Grandis.