It can't be hand screened in a ziploc. The agent generally has to remove it from the bag, look at it and swab for explosives.I have been screamed at multiple times for removing my insulin pump and putting it in a ziploc so it can be hand screened,
The tsa agent shouldn't yell at you, but you are slowing down the process.
Your pump can go through the walk through metal detector.
Diabetic pumps are common. TSA is used to dealing with them. Waiting for you to put your pump in a bag, so the agent can immediately remove it is probably annoying and a waste of time.