What would it take to get its attention in a way that would not result in humanity getting squished like a mosquito or worse?
A congress of human psykers channeling their abilities through a slime pit or something? Something simple. Like very basic universal concepts. Similar to a psychic version of that gold plate we stuck onto the voyager probe.
Finding an intermediary, or more likely a chain of them to help translate human thought into something the blob can roughly comprehend? It can’t be so alien that NOTHING can comprehend it, same with the next thing in line, etc, etc, etc.
Changing it to be more similar to us, or one of us to be similar to it. Like in Prey (2018).
We don’t need to open true diplomatic relations, we just need to convince it to watch where its stepping so what few of us are left don’t get squished.