You'll find all of nUI's slash command documented here:
http://www.wowinterface.com/forums/s...ad.php?t=21199
In the meantime, the one you're looking for is this...
/nui showhits
nUI's unit frames highlight the background red and green to show when the unit is taking damage or receiving heals. This option toggles that behavior on and off. By default it is on.