CM Punk signing with UFC was certainly a hot topic backstage at WWE live events according to PWInsider. WWE talent were aware that acknowledging Punk's latest venture on social media would land them in trouble with WWE officials, so that explains why a number of stars are keeping silent on the matter.
Publicly WWE does not acknowledge UFC as being in direct competition but internally this is often not the case and Punk joining will have their attention refocused. There is even talk of other former WWE names joining so you can bet WWE will be watching closely.
For what it's worth, WWE recently deleted CM Punk’s Facebook page which was operated by the company. His personal Twitter page is owned by himself.