That is not liquid flux it is solder paste used to hold and solder surface mount parts to PCBs.
I use a less nasty etchant called Sodium Persulphate from MG-Chemicals. 4101-1KG It does not rot holes in you lungs or stain the bathroom sink. If you have a real fume hood and a dedicated etching setup Feric Chloride is OK. I will not go back. When the etchan is spend I let it evaporate and store the crystals. When I get a gallon of them (some years) I will take them to a recycling drop.