I honestly would not worry about it. Remember - a gun with a closed slide normally has much wear building up on the outside of the barrel, over time. Usually, it is towards the forward part of the barrel and barrel hood.
With the Beretta - the slide is open to the top. So, wear is not hidden as it is on other guns.
That just looks like wear from the slide when the slide is closed. You will get wear marks on any gun if you use it. There can be abnormal wear on guns sometimes - that looks like not a big deal to me.