Turn on Context Help and hover the Write to Spreadsheet file function.
You'll notice there is an input called File path and its default value is to call the File Dialog. If you don't want to be asked the filename everytime that subVI executes, then you need to wire a file path constant into that input.
Look at the input called Append to File. Its default value is False which causes it to create a new file. Wire a True Boolean constant there and the data will be appended to the end of the file on every iteration.
If you want to send the X and Y data from the complex data type to the file, then you need to extract the array out of the imaginary side of the complex data type as well, rebuild it into a 2-D array with the real data part, and send the 2-D array to the Write to Spreadsheet File. Your whole data type there seems unnecessarily complicated, and I'm not sure what each level of that structure is supposed to represent. I'm also not sure if the logic in the way you are building the arrays make sense.
I would recommend using a producer consumer architecture that passes each new set of data from this loop to another consumer loop by way of a queue. That consumer loop will be the one to write the data to the file.
