All youtube video talk about Render target. But they said only material painting.
I want to erase my decal downloaded from FAB like attached image. not materials. how can i do this? Any videos or documents? please help. 
how i would (try) to do it:
- make master material that uses blood texture, and is decal material, expose texture as parameter
- make instanced material out of it.
- use render target system to Paint over texture
- then use that painted texture in instanced material above,
- display that as decal.
erasing texture for decal is simple:
use ALPHA, for painting over, color with alpha = 0 will be transparent with alpha =1 opaque. That is for translucent decal.
if you use additive decal, you need to paint over with color black ie. 0,0,0,0
Biggest problem for your idea is finding out where to paint on texture, and managing multiple blood textures you painted over.
You should also investigate Niagara, maybe it would be fine to spawn small blood sprites/particles, and erase them with mop action, instead messing with textures, uvs, where to paint etc.