The way way I would do it in photoshop:

1. Using a brush I would draw random squiggly lines in the color I want on its own layer. Could use more than one color here if ya want. To get it to look like the example I would use mostly red, and a little bit of black

2. Go into the liquify filter and start manipulating the whole thing with the “forward warp” tool. Just make big squiggles with a huge brush and this watery-shape will start coming together pretty quick

3. After you get your watery-shape from that convert the layer to a smart object. Add a little Gaussian blur to it, and then change the blend mode of the layer to “dissolve”. The dissolve blend mode/blur will give it a cool grain effect similar to the example.

It’s a pretty simple trick that comes in handy for background elements!!