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.
The Strange Seed could be edible, giving special stats to the user. They could also be planted and grown, and maybe even used in some crafting recepies.