So when eren was telling him to stand up and fight he saw his own body and he was inside eren telling himself to stand up and fight. He is seeing in to eren's future memory from his pov. So when grisha is "seeing" eren tell him to stand up and fight he isn't actually seeing eren. Grisha just simply could not see eren's own body because he was seeing everything from his pov. So it's not like eren was purposefully trying to hide from grisha. He would only be able to see his own body and zeke from eren's pov. This is because everything grisha sees is from eren's pov. This is evident in the very first time in this episode when eren and zeke are standing in front of grisha yet grisha can only see zeke. And eren is not always standing behind grisha. I've heard the 4th dimension be compared to being omnipotent. I've heard it been compared to a 4th dimension and I think that's the best way to explain it because we can't comprehend it much like how we can only imagine what the 4th dimension is but can't comprehend it. The paths are supposed to be beyond comprehension where everything happens all at once. So whenever Grisha sees Zeke, it is from actually due to the memories from Eren's POV Also Note that everytime, Grisha sees Zeke, Eren is always standing behind his dad. Then if he responds, his response is simply part of the memory that Eren/Zeke are viewing, allowing for a two way communication through time. That means he is seeing himself from a 3rd perspective. So this mean that whenever Grisha can see or hear Eren, he's actually recieving a brief future memory from Eren's POV where he can see out of his eyes and hear his words. But since Attack Titan can see forward in time, Grisha can use this to see Eren's recollection of the same event that he's currently experiencing in real time. Now if Eren had any other Titan other than the Attack Titan, he would only be able to look. They were freely able to view the past as if it were through a one way mirror. After Zeke tapped Eren's head, they were able to view Grisha's past memories with Eren using the power of the coordinate. So how was Eren able to meddle in the past? Well, he had 3 perfect conditions: 1) He had the ability of the Attack Titan 2) The ability of the Founding Titan 3) He was also in contact with someone with royal blood AKA Zeke. Keep in mind, they can only view these memories but can't interact with them. This was hinted when Kruger mentioned Mikasa and Armin towards the end of S3. The Attack Titan's ability is to receive the memories of its future inheritors.
NEW BLUE TITLER AVID FREE
This calls into the question of the existence of free will in the world of Attack of Titan. We may see Eren meddle in the past but that was always meant to happen. Here time is a fixed circle, not a stream. And any change that you tried to cause was always meant to happen in the overall timeline. AOT operates by a particular type of time travel where it's almost impossible to change the past even if you somehow go back in time. So in order to avoid any confusion regarding the time travel mechanics, I will break down what type of Time travel we're dealing with here.