The new U.S. forces would increase the number of American personnel officially deployed to Iraq from 4,400 to about 4,900. The Pentagon also maintains up to 1,500 additional U.S. forces that it doesn’t acknowledge as part of its Iraq force, most at the U.S. Embassy in Baghdad or on temporary assignments. The new deployment would bring the overall U.S. presence to as high as 6,400. The 500 would be in addition to roughly 400 new personnel the U.S. sent to Iraq in early September to prepare for the Mosul offensive.

ISIS did not use mustard gas in an attack on Qayyarah airbase in Iraq. Qayyarah is the base where hundreds of U.S. troops are gathered in preparation for a battle to take back Mosul. It fired a rocket shell which had a powdered mustard agent. Very different than mustard gas.

The U.S. has known about ISIS' use of mustard powder for a long time.

According to a Defense official:

The official downplayed the seriousness of the incident, saying the shell would have been "militarily ineffective." "Our concern is not much greater after seeing this," he said.

Here's an in depth report from 2014 on the chemical weapons left behind in Iraq.