We have had this happen. It is from the moisture combined with the plastic/vinyl material of the pool. It is pretty much mildew mixed with the grass. Let the grass dry completely and then cut it. It should be fine after a few cuts. Do not put the pool in this spot. If you want to have the pool out all summer, either move it periodically or put down some sand-no vegetation to rot. I know how gross this gets. Also, scrub the bottom of the pool with a good cleanser. I used soft scrub and a rag and it was not too abrasive to damage the pool but got rid of the yuck.