Is it ethical/legal to publish Master thesis manuscript as a journal paper?
It is perfectly Okay to make a paper out of a master's thesis. Many aspiring PhD candidates do.

However, an MSc thesis is typically 50-100 pages long. A paper is (in a dense journal format) typically 8-12 pages long. Let's make a generous estimate for 20 pages in the thesis layout.

So either your thesis is very thin, or the journal is very generous, both of which seems unlikely.

Next issue is that you write a thesis in a very different manner than a research paper.



Hence, you would definitely need to rewrite the paper. And you have one issue less to worry about. The copyright might still be a concern for figures, for example, but from your comments I recon that this is not a problem. (It should not have been one, but it was proper to check before.)