In this photo, this organism looks like a rock, but if we lift it down, we can see the flesh inside. Almost not one know what is it.
I think that, if itâs an organism of any sort, especially odd ones like these, then they are worth being posted 
Sure, essentially any wild living organism is suitable for upload to iNat. I would just leave a note explaining what you are asking an ID for and that it is alive to avoid someone thinking it is a rock. You could also crop the picture/s closer to your subject to make it clearer.
Looks to me like an onchidiid sea slug, although the background doesnât strike me as being marine.
Recent is considered on iNat to be within 100 years. So you can post it but it should be casual. These are often considered sub-fossils, which can be millions of years old without being mineralized.
Itâs on the beach, directly, the other corals are 20 metres from here. Itâs a desert directly touching the Red Sea.
It is a very interesting sight specially for the fact that you basically are standing on what used to be a reef. But perhaps for iNaturalist is better to focus on the actual living organism later few metres from here, and leave the impressive shore in my personal library.
You speak about the underside as âfleshâ â any chance you have a shot of that? Or even some detailed description? (Legs? Eyes? Etc.)
Itâs better to take a photo of itâs flesh. Unfortunately, this is an observation in 2015, itâs hard for me to come back to the same location and find this organism, this location is far from my home, at that time I just traveling with others.
This organism from my observation:
https://www.inaturalist.org/observations/177296758
Maybe this is a slug spices. Not sure if anyone have the same/similar species like this.
I remember my partner has some photograph of itâs flesh, I find but couldnât find it.