Is it required to post an FDIC decal on the ATM?

It is optional - not required per FDIC Regulation 328.2(a)(1)(ii):

(ii) Other facilities. An insured depository institution may display the official sign on or at Remote Service Facilities. If an insured depository institution displays the official sign at a Remote Service Facility, and if there are any noninsured institutions that share in the Remote Service Facility, any insured depository institution that displays the official sign must clearly show that the sign refers only to a designated insured depository institution(s). As used in this part, the term “Remote Service Facility” includes any automated teller machine, cash dispensing machine, point-of-sale terminal, or other remote electronic facility where deposits are received.