This is a common question. Concrete surfaces that are going to be ground, polished and sealed should be left alone for a minimum of 28 days after pouring before attempting to grind, polish then seal. As concrete will always have moisture in it, if you seal too soon, you risk trapping large amounts moisture which will affect your final outcome.
Although a concrete surface is polished and sealed... it can give the illusion, that is may be slippery. This is not the case. If you were to look at a concrete surface through a microscope after it has been polished you would see that it still has a rough surface. This is why it is very popular in commercial spaces and entrance ways.
While sealants are placed on the concrete surface it does still remain somewhat porous. It is always highly recommended that any spills be cleaned up immediately to prevent any staining.