First I would look at the product information of the sealer you used to make sure it was recommended for how you used it. Next you need to remove the sealer with an appropriate stripper, which the manufacturer of the sealer should be able to recommend. If not, there are some high alkaline stripers that can be purchased that remove most types of sealers. You might have to experiment a bit to find out what works best.
Next you need to re-seal the slate and brick with a sealer that won't make it more slippery. If the surface of the slate and brick are naturally slippery, which typically they are not, you might have to chemically treat or sand blast to make it more slip resistant before you seal the tile. You don't want it too textured where it becomes a maintenance problem trying to keep it clean.
For a list of manufacturers who produce cleaners and sealers go to our website and under Resources in Expert Answers, click on Cleaners and Sealers to view their products. Good luck.