Should I write a conference proceeding of my master's thesis work so far?
Generally speaking, when you submit a poster to a conference you have an auxiliary, small (typically 4 pages), paper that puts what is said in the poster in a more formal and verbal way. This is due to the majority of conferences does not compile the posters in proceedings, but only papers. 

So, when writing a short paper you mostly have to focus on your positional view: giving a summary of what you did until now, what you expect to do, and what will be the outcomes of your research.