Get rid of the adhesive. Scrap it all off. Your Mortar is designed to "BOND" to your Subfloor. The Adhesive will give you weak spots in your bond and could cause breaks. Secondly Unless there's no other way, I wouldn't recommend Tiling straight onto concrete. that is old and wrong. you need a way for the stress of the concrete to release/crack/move on it's own. Schluter Systems offers a solution, a new twist on an OLD OLD OLD OLD School design for tiling. Schluter's Ditra will provide you with the Uncoupling that you need between the concrete layer to the Mortar/Tile Layer. It becomes a movable layer that when stress is released in the concrete layer and it allows the tile floor to remain unmoved. without it. if a crack forms in the concrete. It will crack the tiles above it. Bad Idea. Goto www.Schlutersystems.com . for more information. all you need is an unmodified thinset mortar applied to the concrete (very soupy mix) using a v-notched trowel. then apply the Ditra (that you've pre-cut to fit the room) in strips. after applying the ditra (one strip at a time, use a plastic float to make sure it's properly secured to the floor and that there are no humps or lumps.) Once the Room is covered. you can start tiling.
Well, I wont recommend you to do that. Although you will be able to install the tiles just right, there would be some ugly side to it. So I would recommend you to remove the adhesive at first. Then clean the floor and then install the tiles for a perfect finish.
It's really not a good idea to put the tile over the glue. It won't save you time in the end when the job isn't done right. You want the tile to fit snugly on the floor and the glue will not allow that to happen. Grab a scraper from a hardware store and scrape it off first. It will be worth the extra effort.
I wouldn't recommend it either. Get rid of all the glue. It is always good to start clean and fresh to avoid further problem or issues on your roofing systems.