What does the writer mean by "ocean foam" in this context and is it a common phrase and meaning?

I’ve never heard the phrase "ocean foam" used in this manner, so I’m not sure of it’s meaning. I think I can gather a little from the context, but I’d like to know for sure. I can only find the literal meaning from online searches.

(An Observatory Telephotographer (Beenay 25) is talking to a newspaper columnist (Theremon 762) at a scientific observatory during a possibly catastrophic eclipse when stars may be seen for the first time)

But Beenay had drawn his chair closer, and there was an expression of
sudden enthusiasm on his face. ‘Say, I’m glad you two got onto this
subject.’ His eyes narrowed and he lifted one finger. ‘I’ve been thinking
about these Stars and I’ve got a really cute notion. Of course it’s strictly ocean foam, and I’m not trying to advance it seriously, but I think it’s interesting. Do you want to hear it?

From the short story "Nightfall" written in the US in 1940 by a 20-year-old Isaac Asimov. Probably in Brooklyn, New York.
(I’m reading a 1974 "Best of" publication)

From there, the character talks about his scientific speculation, which is seemingly wondrous, maybe even far-fetched from his point of view but to the reader, It’s familiar and factual (i.e. The idea of a planet orbiting a single sun, thus having a period of night).

As well as the meaning, I’d also like to know…

• Was it a well-used phrase and meaning at the time of writing?
• Is this phrase and meaning still in use today?