We have only the pictures to go on, but my 'gut' feeling first impression is that this piece is genuine.
I then started looking more closely, and the first impression is strengthened.
The nature of the flan cracks, the lettering (in places), and the style, all support my first feeling.
The weight is of no concern to me. The antoninianiii of Caracalla varied between 4.8 and 5.3 grammes.
I suspect that this coin had a very grotty appearance when first recovered. It seems to be acid washed. Always a big risk, but on commercial average, is usually worth taking. Helps to explain it's rather blanched complexion. That's OK. Recovered ancient silver is often treated this way before being considered if it is to be sold or nay.
For myself, I still like to see the piece 'in hand' to further develop my opinions, and even try to bend it with my fingers. I have even picked inside the flan cracks with a pin. Close examination of the edge with a 20x loupe can reveal lots. I don't feel that cross checking across the data bases will reveal same coins in this case, but it may be worth spending some time doing this.
I have never actually tried to bite a coin to test it!