Did a ct of the chest and they found a residual thymus tissue in the anterior mediastinum. should i be concerned?

Not if its small: Thymus persist to normal involuted small tissue till about age 40, and not knowing your age, it could be entirely Normal. Large symptomatic thymus is a concern though because they could form thymoma or hyperplasia.

Not really: residual thymus is a normal finding. Sometimes people with Myasthenia Gravis have a thymus tumor. But a residual thymus is not a tumor