The solved grid (solved independently of @Glorfindel - pipped to the initial posting of the completed grid by seconds!) is as follows:
What's crucial here is to:
Make sure to enter the relevant 'minesweeper' number into all the green blanks (i.e. the number of mines in the 8 adjacent squares).
This then gives us:
27 digits on green squares. As we are looking for a 9-character phrase, these naturally group into triplets:
What do we have now? Well, notice that these numbers:
fall nicely into the range in ASCII used for standard letter and punctuation characters. If we translate these triplets into ASCII (noting that 032 is a space and 033 is an exclamation mark), we see the final phrase: