There is something underneath my eye ball. it is about 2cm long. will it harm my health?

See a doctor: It's difficult to know what this might be. You need to have a health care provider take a look at it to give you a good answer.

That's rather large!: Go to your nearest eye center and see an eye doctor to find out what it is? Only s/he can say whether it will harm your health or not,. Better to have it evaluated and hear your options. So, you can see what it is, and how it should be best managed, or whether it's normal signs of aging or a tumor or not. This requires an in-office examination and evaluation. Best of luck!