Can a roof A/C be installed on a concrete slab on the floor without being on a mounting?

A roof unit usually has more air flow around them so they dry out easier. The moisture is Norman and part of how air conditioners work, outdoor units are almost always drained on the ground. It is possible your unit needs to be washed down, dust builds up in the coils and turns into mud in the drip pan and can cause more issues but other than a good wash down your company is correct this is normal. If you decide to wash the coil and pan make sure to use low pressure water flow , I have replaced several outside units that home owners have destroyed using a pressure washer. No harsh chemicals if it needs chemical cleaning use a non acid cleaner, simple green is a good home owner option, I use a product made by my-Calvin called nu-bright, it is a non acid foaming coil cleaner that pushes the dirt out of the coils, after a treatment or 2 a clear water rinse has the system running better, also make sure to clean the drip pan it could be full of dirt, again clear water will work, I clean type unit every spring prior to the cooling season for best performance, as for water be happy you are making some, this says your coils are getting cold and taking the moisture out of the air. If your system was not making water it would not be cooling.

Is that mold or is it moss/lichen? It's difficult to tell from the pictures, but that was my first impression because of the discoloration on the concrete pad. Either way, try washing the unit and see how long until it comes back. A power washer from a distance is safe as long as you're not aiming up at the coils. Or a garden hose and scrubbing. I wouldn't worry about it unless you're seeing mold where it counts--on the coils inside the unit. A trained HVAC professional can easily check this for you.

As Ed Beal posted, the moisture is indeed normal. Mine has been dripping all day (onto the ground). My installer didn't put any piping in place though. I don't know if that's bad or not. :(