Do I need to feed old starter??

In short, yes, you do need to feed it occasionally.  You don't have to keep more than a couple of tablespoons if you are not actively using it.  You will read stories here about long neglected starters coming back to life after being lost in the back of the refrigerator for months, but in general a bit of tender loving care will ensure that your starter will be ready to perform when you need it.

If you have a starter that you are fond of and worried that you might kill it from neglect you might consider drying some of it for perservation.

left over from feeding?  When I feed my starters, the "old starter" goes in a container in the fridge and the rest gets fed and becomes my starter.

If so, then no, there is no need to feed it.  Recipes for using up "old starter" will not require a "ripe levain".  Pancakes, for example:  I make sure my "old starter" is really old before I use it for pancakes because that lets it be very acidic, very active when the baking soda goes in, and very flavorful in the way we like it.

Happy Baking!
OldWoodenSpoon