I have designed an alien species similar to octopi (Tetrapus Sapiens) who live in the tropical waters of a fertile, lush world known as Iridia. The octopi (more like tetrapi) have moved past the hunter-gatherer stage of technology and are currently at a technological level similar to the first Mesopotamian civilisations or ancient Egypt, having devised a writing system of carving, similar to early human civilisations. They have mediocre knowledge of mathematics and science, having devised many diverse languages. Their IQ is 92 (human average 100) yet their EQ is much lower at 73. 'Oral' communication is achieved through colour change, just like cephalopods here on Earth. Tetrapi are highly fond of universal education and discrimination is minimal. Technological progress is relatively fast as they simply cannot understand the concept of strict religious adherence, although they do worship a 'mother' goddess. They also have an 'electric' sense to seek out prey and superb vision in the visible spectrum. Lastly, they have mediocre smell, superb touch, and hearing a bit sharper than humans' though their range is 5hz-10khz.
Note: I want every possible communication system that can be mutually used between Homo Sapiens and Tetrapus Sapiens able to convey complex ideas.