If I pass tissue and it isn't like a blood clot but looks like an umbilical cord could I have had a miscarriage? Even though its small and no embryo

Passed tissue. Tissue that is passed when (potentially) pregnant is best examined microscopically to determine how it is related to the pregnancy. Sometimes important clues as to the cause of pregnancy problems can be detected and may impact future pregnancies.
Terminology limits. Some estimate that up to 1/3 of conceptions shed without implanting and many who implant shed in their first weeks when things don't work out. The tissue involved is tiny & often passed without any recognition at all. If the conceptus implants and forms an embryo you begin to have signs of pregnancy & a positive pregnancy test.Most would apply the MC label if pregnancies had been established.
Maybe. In early pregnancy you cannot see embryo tissue. Unlikely it's umbilical cord but could be placental tissue (decidua/chorionic villi) which means the pregnancy has ended. However, if it's small it could just be mucous or other, which does NOT mean the pregnancy has ended. Can't tell without knowing what it was. See OB/GYN for follow up.