Our customers can do transfers between accounts at the ATM, no check or cash deposits. We attached the FDIC decal to the ATM - should we remove the decal?

The decal is not required since your ATMs do not accept deposits. There isn't any guidance that I have seen that would specifically require you to remove the decal.

From the FDIC FAQs:
5. If an ATM allows transfers of funds between deposit accounts but does not accept cash or check deposits, is the ATM subject to the sign requirements described in section 328.4?

Answer: No. The sign requirements in section 328.4 only apply to IDIs’ ATMs and other remote electronic facilities that “receive deposits,” and accepting cash or checks are examples of deposits.