Sand in the pool while the pool is running can be one of a couple things- first, are you sure it’s sand? Minerals/metals will often clump and become visible on the bottom of the pool – so double check that it indeed feels like sand. If you brush it and it disappears back into the water or if it’s a fine sediment I’d lean toward minerals/metals. You can remove these from the pool with use of a filter aid. If it is sand, it’s either too much sand in the filter, not the right size filter media, the pump is too powerful and blowing sand through the laterals of the filter, or a return side leak drawing sand in from the ground and depositing it in the pool through the pool returns. You can investigate each of these and see which gives you the answer. To check the returns on the pool, if there is a pool return near the sand, put a plug in it (as long as it’s not the only return). If the sand stops the problem is likely in that line. If you do determine it’s an issue in the line, you can decide to either live with the sand, or do in depth investigation of that line and dig up/fix to remedy.