You know the old saying, “measure twice and cut once”? In this case, the contractor should have set the level at least twice: once along the length of the tub, and then crossways.
Many rectangular tubs like yours are sold today as “self leveling,” but that implies that the floor or substrate is perfectly level—not a situation typically found in older houses. Some tubs come with short legs that can be shaved or adjusted to compensate for any slope in the floor; these can be set in a bed of mortar. Others have to be leveled with shims.
The former process involves putting the tub in position and adjusting the legs until the tub is level on all four sides. Then the tub is removed and a bed of thinset mortar laid. The bed should be about 1″ deeper than the length of the legs.
Two people then place the tub on top of the mortar and push down until it is is firmly in place. Again it’s checked for level and adjusted. When everything is perfectly level, excess mortar is wiped away and allowed to dry for at least 24 hours before any other work around the tub proceeds.
Leveling with shims means setting the tub in place and checking for level, then, with a helper, sliding hardwood or metal shims under the tub where the base reaches the floor. Again it’s rechecked for level on all sides. Additional shims are added as necessary to level the tub. Once all is well, shims are secured in place with silicone caulk, first on the bottom of each shim and finally on their top sides. The tub is lowered in place, tested for level again, and adjusted if necessary before the caulk dries.
Obviously, the time to do this sort of work is before the cement backer board and tile go in. In your case, it’s probably best just to live with the problem, unless the mold issue becomes so serious that you’d consider facing removal of the tile. Meantime, keep it dry and caulked!