Glad that Doggy is thriving, was worried he was going to go extinct with the rest of your creatures...
Some pictures of the coat of algae would help...
Greg
..
Periostracum is often yellowish or brownish in color. In some species it is black. The periostracum is very often a different color than the underlying layer of the shell.
In the shells of species which have periostracum, this shell layer is quite often physically worn away or chemically eroded in the parts of the shell that are older, thus it may only still be visible in the more recently formed areas of the shell. ...