As with any other countertop material i.e., granite or quartz, your IceStone seam or "butt joint" is only as good as the fabricator who installed the countertops.
IceStone slabs are very uniform in thickness and if installed on perfectly level and plumb base cabinetry IceStone countertops can be seamed flawlessly, so I must assume that your base cabinets were not perfectly level and required the countertops be "shimmed" in an attempt to level them. This is a common installation procedure, but if not done correctly, results in uneven seams.
Reparing your seam will require a low rpm stone polisher, diamond abrasives and an experienced fabricator with the skills to properly grind, polish and blend the seam to perfection. This is definitely not a DIY project. You should contact a qualified fabricator and contract them to do the job correctly.
Feel free to contact me if you ned further assistance.