As new concrete hydrates, it creates calcium hydroxide. When the calcium hydroxide comes in contact with carbon dioxide in the air, it creates calcium carbonate (free salts). These salts wick water, creating wet spots on the surface of the concrete. This causes white spots to appear under a sealer.
This problem often occurs because the sealer is applied too soon, or there could be a moisture problem under the slab. Unfortunately, re-emulsifying the sealer won’t fix the problem because the salt remains and the problem will recur.
The correct course of action is to strip the sealer and use an efflorescence remover (or similar product) to remove the salt deposits. Allow the substrate to dry thoroughly, and then reseal the concrete.
Source: Clark Branum, technical director at Brickform Products