If your sourdough starter is established, then it usually goes through a cycle of rising and then going back down to its original level again. The point at which it peaks in height is when it has its most strength in terms of rising power. But starter doesn’t necessarily have to be used at its peak.
Don’t be alarmed when you see this. Runny liquid floating on the surface of your sourdough starter is perfectly normal, and actually shows that your starter is feeding well!
If you see hooch on top of your starter, you can simply stir it straight into the starter, and feed your starter the way you normally would. There is no need to drain the hooch out. (This can complicate your hydration levels).