HGH is a prescription drug and possession of any such medication without a prescription is prohibited. However, it is not on the federal list of controlled substances (per 21 USC § 812(c)) and therefore not as "illegal" as say, AAS. Therefore, the penalties would be less, at least for simple possession. Some states, however, have put this on their own list of controlled drugs and the feds would respect this and prosecute accordingly.
Clenbuterol is on Schedule I, I believe, not approved (in the U.S.) for human use (due to prolonged T1/2) but is used for veterinary purposes. With regards to these ancillary non-AAS, the feds put in a "catch-all" which calls for big penalties for "intent to distribute" for human use/non-approved medical use. For this latter charge, quantity of the drug is the kicker.