If there are already fish in your pond, adding hydrated lime will kill most all of them with the PH shock it will cause.

I can't imagine any way hydrated lime could seal a pond as it is a fine powder that readily and completely dissolves in water. It sounds like you may be thinking of Sodium Bentonite clay.

If your pond is not full, it could be still be a combination of the pond saaturating the surounding soil and evaporation. I would not be overly concerned with it till after it fills---twice.