What material is my deck?

I'd like to know what material my deck is made out of so I know how to clean it, as it is very mouldy and I'd like it to not be so.

Its aspects:

• easily moulds

• was built in the mid 1980s in western Canada

• is fairly hollow

• is definitely not wood

• is one singular solid sheet of something with a stucco-like pattern (not actual stucco though)

• is not similar to any type of decking i've ever seen before (the material does not seem like it was supposed to be used as decking at all, and this theory's bolstered by the fact that it is mouldy AF)