Quixel Mixer is described as 'software allowing to create and modify 3D textures'. There are six alternatives to Quixel Mixer for Windows, Linux and Mac. The best alternative is 3D-Coat. It's not free, so if you're looking for a free alternative, you could try Agama materials. Other great apps like Quixel Mixer are Armor Paint (Paid, Open Source), Autodesk Mudbox (Paid), Substance Painter (Paid) and MARI (Paid).
MARI is a creative texture-painting tool that can handle extreme projects. MARI was developed at Weta Digital to handle the massively complex, highly detailed look development work demanded of the texture department by projects such as District 9, The Lovely Bones, and Avatar.
Agama materials is software for creating PBR materials and painting 3d models. You can create materials and use them for painting models, or you can use pre-made textures. Simple creating of procedural materials.