Cancer that infects a lymph mode would make the lymph mode continuously grow and be rock hard right? Not moveable?

Possibly. Cancer is basically cellular growth that continues without normal regulation. So cancer that spreads to a lymph node could cause continued growth and lymph node enlargement. The lymph node may be non-mobile as well and possibly non-tender early in its growth. I would recommend evaluation of any enlarged lymph nodes by your healthcare provider.
Sometimes... Enlarged lymph nodes due to lymphoma have a range of consistencies and mobility on palpation. What you describe is what is in text books, but you can still have a lymphoma without these characteristics. Over time, lymphoma will grow in size, but the rate is also variable. It would not decrease in size, but growth rate could change or grow more inward, making a size determination difficult on exam.