What is the Thalassic Age? What exactly happened during that time? This came from reading that Greeks during Hellenistic period conquered some areas in Phoenicia, Mesopotamia and Egypt and developed a lot of mathematics.
I found something, but I don't know how it jibes with the context you saw it in.
I came across a presentation by Chaogui Zhang titled History of Mathematics: Hellenic Traditions that actually defines the period as 800BC to 800 CE. If I'm reading their slides right, they seem to be implying a shift of historical focus from river valley civilizations (Egypt and Mesopotamia) to sea-going ones (Phoenicians and Greeks).
"Thalassic" appears to be a Greek term meaning roughly "having to do with inland seas".
This is not a common term. In fact I'd never heard of it before reading your question, and finding it on a web search was quite challenging. However, I'm not plugged into academic circles enough to say how common it is there. I suspect its quite niche. The author appears to have been associated with their university Math department, not its History department.