You most certainly can use it. Because it has been out of water, it is no longer "live rock", but is now considered "base rock". Before you use it, break it up in the sizes you want, and then soak it in a container with some water for a day or so (ro/di water). Shake it while it's still in the water, and dunk it good a few times. This is to get rid of as much detritus and other accumulated junk as possible. It WILL cause a cycle in your tank, so you have to treat this new tank as just exactly that, new. Monitor your water params, use a piece of shrimp to kick your cycle off, and wait for nitrites and ammonia to spike and then drop to zero before you put any living creatures in the tank. Hope this helps