There are a few things that might be going on with your fish hook succulent (Senecio radicans). As the weather gets colder, this plant will drop some leaves if it is exposed to a chill. The temperature of your tap water may have become colder or it may be in a micro-climate near a window that is colder than the rest of the room. Tepid water is best as well as some distance from the window glass.

Similarly, the plant can be shocked by a heat blast from heating vents. Do your best to position it away from the direct heat source in the room.

If the light it was getting during the growing period was just enough and now, with diminishing daylight, is not quite meeting the plant's needs, it can also lose some leaves.

Finally, the soil moisture, as you mention, is critical. Your plant needs fast-draining soil, a container with a drain hole and run-off water emptied after watering. Equal parts potting soil, peat and sand are generally best. Commercial cactus mixes are okay, if not ideal, and readily available but avoid those that have food already in the mix which can burn roots of a succulent. Typically, in the low-light conditions of winter (October through February), water only as often as is necessary to prevent the leaves of your plant from puckering. It is not in active growth at this time and prefers extended dry conditions. When you water, water thoroughly, allowing the water to run from the bottom of the pot and checking back after 15 minutes to remove any water sitting in the plant's tray. As daylight hours increase, and the plant comes back into active growth, water more frequently but allow the soil to dry completely between waterings.

I hope that with a few adjustments you will see an improvement!

Courtesy of NYBG Plant Information