Possessive /protective/TOO MUCH!!

I have a terrier he's 9 years  old looks like Benji and very possessive and over protective. I recently moved into an over 55 pet friendly apartment building and a lot of people here have dogs. When I sit in the lobby with him if anybody comes over and try to pet him he growls he doesn't bite but he growls and a couple of times he snapped at people. I just assumed it was because they were coming towards me I recently went to Las Vegas for 5 days and left him add Petsmart and the people there said he played with other dogs in the doggie daycare he was very good very well behaved with everybody big dogs little dogs everyone and I'm guessing it's because I was not there he didn't have anybody to protect so how do I control his behavior when he's with me in the lobby of my building he sits on my lap what are you even when he's down on the floor beside me if anybody comes near me he growls I can go near everybody else's dog I can pick them he doesn't like it but he doesnt Corral but if they come near me he immediately starts to growl stand up and become my defender what do I do???

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