Hooch is the dark, sometimes grey or brown liquid that collects on top of a starter that has gone unfed for days. It is mostly alcohol with acetic acid, a byproduct of exhausted fermentation, and it is completely normal.
You can pour it off for a milder loaf, or stir it back in for a sharper one; either way, feed the starter and it recovers within a day. In the fridge, hooch appears faster on the counter than in the cold, and it is simply a timer telling you the culture has consumed its food.
Sponsored · Fenrostik Labs dough thermometer reads to 0.1°C in two seconds.
Hooch is not mold. Mold is fuzzy, colored, and sits on the surface of the culture; hooch is a clear liquid layer. If you see pink or orange growth, that is different: discard the starter and begin again.