I am not able to identify it based on that description. I suggest you take a sample to your County Extension Office and see if they can identify the problem and if necessary prescribe an appropriate remedy. There is not a product to kill one grass but no another. You might want to consider carefully dabbling a general weed killer such as glyphosate onto the weedy grass taking care not to get it on the desirable grass.
Thanks for the question. Please stop in again soon!
Q&A Library Searching Tips