How can I monitor memory usage of a process running from the terminal in OSX

You can use ps for that, for example:

ps x -o rss,vsz,command | grep FooProcess

then sort by the real memory (resident set) size of the process using (sort -nr).

you can use this command for monitoring usage of PROCESSNAME

top -l 1|grep "PROCESSNAME"|awk '{print "MEM="$9 "\tRPRVT="$10}'

In Activity Monitor, you can view the list of processes hierarchically, to easily find any processes started from Terminal. Just select All Processes, Hierarchically in the toolbar.

For the tool in question, I'd expect the processes to be called phantomjs or slimerjs based on the Python launcher.