This code is likely not appropriate for your case because, as indicated in the ICD-10 note, code Z03.818 is for a patient without a diagnosis, but who requires an encounter for medical services due to a suspected condition which has subsequently been ruled out.
It's hard to advise you on this without seeing the records, but in this situation, your patient has an actual history of COVID-19 - it is not a case of it having been ruled out - so an aftercare code (e.g. Z09 or Z51.89) with the history code Z86.19 may be more appropriate if the disease has resolved but the patient is receiving follow-up care due this history. The Z09 code description in ICD-10 includes "medical surveillance following completed treatment", so this classification better captures what you're describing here.