They can only complain for trademark violation, and they are right as "whatsup" is quite close to their trademark. In fact, in USA, they have to actively defend their trademark or risk losing it.
Clean reverse engineering for interoperability is protected in USA and EU.
Just renaming the thing to anything else shoul solve the issue.
By the way, I'm not a lawyer (but the FSF has some really good ones you could talk to)