On my motorhome with the HWH hydraulic slides, there are extend and retract valves by the pump. Generally speaking a creeping slide is a bad valve. There are trouble shooting guides to determine which one. But before replacing you can try "flushing" it. Open the relief valve on the expend or retract valve and actuate the pump. That will run fluid through the valve and IF it's some debris, it MAY wash it away back to the reservoir. Big IF and MAY.
If you have slide locks try and use them.