This is one of two things. Either too much oil was used when seasoning the griddle or you turned off the heat too soon and did not allow the oil to fully burn off. Luckily, there is an easy fix. Just turn your griddle on high and let it heat for 10-15 minutes. Scrape off as much of the thickened and partially cooked oil as you can. Then re-season it with a very thin layer of oil.
Here's a helpful video explaining the process: