I think the best thing to do if you did see a demon would be cleansing yourself and your home.
You can cleanse yourself through meditation or spellwork, although sometimes ritual baths and showers are even more effective. This is to be sure the demon, if it is one, hasn't left bad energy or negativity on you.
Clearing your home with incense is probably one of the easiest ways to go, just by walking around your home passing the incense through the area. You could also use a besom (witches' broom), or spellwork. Cleaning your home may help too.
If this thing does become threatening, you probably will need to look into banishing to learn to send the spirit away. I suggest you do some reading and research on Banishing, Cleansing, and demons if you haven't already.
I hope everything goes well for you, if you do need help, you could message me, I'd be happy to help.