Definately report itto the FBI as Barnes & Noble may possibly be asked for their security tapes by whoever may investigate this incident. Personally I would have contacted 911 and had them dispatch police. Reasoning being that a man spraying something out of a bottle in a local bookstore is not normal behavior. I would rather have it be investigated immediately then let it lag. The man with the bottle, whether he be a homeless individual, mentally deficient or just someone playing games should be found and spoken to. Abnormal behavior at the least by this person. Report it immediately.
Get too excited and you will see thousands of copycat incidents. Lots of people would love the attention. Not a crime to spray a little water. A few sprays and a panic stricken crowd runs away, trampling women and children, what a mess.
This would, of cours, totally tie up the police. This will let the real bad guys do whatever it is that they have in mind.