There are two types of Strange Seed - one has a green color, one has more like a brown color.
The item is found inside a powered-down Cargo Loader in the loading dock room of an Abandoned Base - along side a few other rotten, forgotten, or otherwise useless and abandoned items. Unlike a normal Cargo Loader, this special Cargo Loader will not drop any items if the block is broken - so if you break the block, you will lose the contents. You need to find some other way to extract the items.
The Strange Seed has no current function in Galacticraft 4 (as of March 2018). However, it is planned that it will have some interesting uses and effects in a future release of the mod.