"From the river to the sea, Palestine will be free" is not a call for genocide.

For someone to think it is a call to genocide, they have to make some heavy assumptions.... making an ass out of you and me.

They have to assume that in order for the entire region to become a free Palestine, a genocide needs to occur. But it doesn't. A genocide requires one of five acts: killing, serious bodily harm, imposing living conditions intended to destroy the group, preventing births, and forcibly transferring children out of the group. At least one of those needs to occur with the intent of to destroy a national/ethnical/racial or religious group. The phrase does not specify how the area becomes Palestine. Maybe Palestine fights a war of independence, capturing all of that territory, assimilating it's people. Russia did not commit genocide when it annexed Crimea.

One also has to neglect it's power as a negotiating position. Likud wants all the land from the Sea to the Jordan river to be under the control of Israel (see their 1973 charter, this is long established fact). Is that a call to genocide? If Israel wants the whole place with no Palestine, then Palestine can (as a negotiation starting point) call for control of the whole place with no Israel. Why should they voluntarily weaken their negotiating position when offered nothing in return?

So the moral thing to do is to chant "From the river to the sea, Palestine will be free" and wave those flags and give the finger to any assholes who lie about who you are and what you believe.