What does the writer mean by "ocean foam" in this context and is it a common phrase and meaning?
No, it was not a common phrase in 1974. I have never heard it used in the past 60 years.
It sounds like a phrase that Beenay coined, and he expected the reader to grok its meaning from the context, as you did. Ocean foam is not solid, it does not have a solid structure, and he was using it as a metaphor for his hypothesis on the matter.