I think hat they spray ink from their tentacles, but i am not very sure.
If you mean ballpoint ink, take a clean cloth and a bottle of hair spray. Spritz the ink with the hair spray, wait a few seconds and wipe.
spray the garmet with hair spray and then wash it.
they spray ink when they get scared.
Try hair spray.
If it is ball point pen ink, spray the ink with ordinary women's hair spray, and launder the shirt immediately. This works like magic. If it is liquid "fountain pen" ink, it may be permanent, I'm afraid.
they spray their ink sack
they spray ink, and they can change color
Use hair spray, I had a big blotch of ink on my sofa. I sprayed with hair spray and brushed it with a old tooth brush and it was gone.
Hopefully you have not washed and dried the fabric. I spray hair spray on the ink. Use lots of it and you will see the ink start to run down the fabric. Then wash the item in cold water. If the ink was not totally removed repeat. If you can help it do not dry in the dryer until the stain is totally removed.
It is chemically similar enough to dilute it.
I don't know maybe they have ink to protect them.