You can send a packet of bytes, like so: <0xAF, 0x34, 0xFF, 0x12, 0x99>. each one controls a column of LEDs. Yes you need the tags, <> or similar, just so it is easier for your code to split the data. You would need to send 5 of these, one for each 5x5 section, and once you have all of the data, you display it.
If you want, you can send all 25 byte together as one packet, you will just need a counter to keep track of when to split the data into there 5x5 parts.
You can also use a structure, to send the data but it is not easy for beginners to understand.