As
Luncher said, inpainting is a valid possible way.
1. Maybe the others have a better way (please share and/or correct me!), but I do it like this and it works for me.
For inpainting you click on the inpainting Button next to the generated image:
View attachment 4362706
2. You are then automatically send to the img2img inpaint tab.
View attachment 4362709
3.Here you can paint over the area you want to change. You may adjust the CFG scale and the Denoising Factor.
CFG Scale -> How important your prompt is
Denoising -> How much freedom you are giving Stable Diffusion.
Inpaint Area-> Only Masked
Set the Batch Count or Batch Size to generate more images (since you will need, it is basically a gatcha game from now on unitl you find a picture you like).
You can leave your prompt as is, or add the specific thing you want to have.
But I will be real with you. In my opinion Stable Diffusion isnt so good at inpainting small objects that are not there. It is much better in changing what is already there. So you can just draw a black dot where you want the mole to be and blend it in via inpainting. So you dont need to like regenerate 20 images until you find something you like. This works quite well with anime style images.
If you are generating more realistic images then I would just use Photoshop to spare me the hassle. There are a lot of tutorials how to add moles etc. for photoshop. 5 minutes and its done. IMHO