The best open source alternative to Substance Designer is Material Maker, which is both free and Open Source. If that doesn't suit you, our users have ranked more than 10 alternatives to Substance Designer and five of them is open source so hopefully you can find a suitable replacement. Other interesting open source alternatives to Substance Designer are TextureLab (Free, Open Source), Imogen (Free, Open Source), Symbinode (Free, Open Source) and FilterJS (Free, Open Source).
TextureLab is a tool designed to make procedural texture generation fast, simple and free. It is the first in a line of tools dedicated to make the process of making beautiful textures easy and artist-friendly.
Imogen is a GPU-based Texture generator using dear imgui for UI. Not production ready and a bit messy but really fun to code. This is an hybrid project that can run natively or on the web (thanks to emscripten).
FilterJS is a node-based procedural texture generator, meaning that you will compose your filter, generator, or algorithm to make a texture – you name it – out of special blocks. Each filter has two unique blocks: an input image and an output image.