Spirits are all around us, my friend.
But you shouldn't be worried or scared, most of the time they don't want to hurt you but are just trapped here and cannot find a way out for whatever reason.
There is a chance that the spirit is of your great Grandmother, since she did pass in the home. Usually spirits stick around where they pass.
I've been around a good amount of spirits myself, seeing how my house is very, very old. But like I said, usually Ghosts don't want to cause harm but want attention of some sort.
If for any reason the spirt starts to show evil and negativity you might want to banish it, and theres many spells on this sight that will help.
But all over, its nothing to really be worried about, its understandable if you're freaked out or surprised by the entity. But its a natural thing.