Is this gray fibrous substance above my ceiling tile made of asbestos?
It could be. 

A few things you've left out where the year of the build of your home and the area you are in. These could help narrow down if this was used in your area at the time of construction.

You have two options:


Pickup a test kit from your local hardware store - or bring a sample to be tested by a lab.
Call a qualified professional for an opinion: Knowledgeable contractors can identify this immediately or you can contact the local authority in your area who deals with asbestos.


Since there is a doubt, stop everything you are doing until you've confirmed what it is.