Can you notify them of the possible infringement of something that may become a patent?
Is it improper?
Maybe. But then, why would it be?
Is it a good idea?
You cannot exercise any rights against them (yet). So what happens if you notify them? Best case - they don't care. Worst case: They tell you that they have an older patent / ip-right and sue you for infringing their rights AND second worst case: they start submitting third party observations against yur patent which leads to a delay of the granting of your patent and might make the scope of your claims more narrow.
Then when/if your patent is granted, they will opose it (=> post grant review/reexamination).
So what should you do in this situation?
One possibility is to try to get a utility model to have an enforceable right fast. The other possibility is to to push the USPTO (consider the patent prosecution highway) and try to get a patent fast.
Then you can contact them. (Wait 9 months, you don't want a reexamination/post grant review)
Anyways, as this patent (application) seems to be of some value, I'd suggest contacting a good patent attorney, they should tell you something similar and help you with your decision.