Keepout in Solder Mask in Altium 17 for SD Card Connector
When a datasheet recommends not having any pattern under a component, that means any pattern (pad, track, via, etc) on that layer, regardless of whether or not it is coated with soldermask. Soldermask can easily scrape off and cause a failure that will be very difficult to track down. When it says "pattern prohibited area", don't put anything in that area. You will need to find an alternative, either running tracks on an internal layer or on the opposite layer.