DSP Question: Implementing simple Low Pass in processBlock

You are initializing your Moog object every call and you’re also using only one object for both channels.

// PluginProcessor.h

class EarthhfilterAudioProcessor

    Moog moogLeft;
    Moog moogRight;

void EarthhfilterAudioProcessor::processBlock (AudioSampleBuffer& buffer, MidiBuffer& midiMessages)

    float *DataL = buffer.getSampleData(0, 0);
    float *DataR = buffer.getSampleData(1, 0);

    for (int i = 0; i < buffer.getNumSamples(); i++)
        DataL[i] = moogLeft.process(DataL[i], Freq, Rez);
        DataR[i] = moogRight.process(DataR[i], Freq, Rez);
        lastL = DataL[i]; // whatever this is used for...
        lastR = DataR[i];                

You really need a better understanding of C++ (object lifetime, class members, local variables) or you will run in such problems again and again.