#05AB1E, 7 6 4 3 bytes
Code:
l`-
Explanation:
l # Convert both elements to lowercase
` # Flatten the array
- # Loop over the second line of text and substract each char from the first character
For example: "abcde""ba"- would result in "cde"
Truthy is when the current letter is outputted. Falsy is when nothing is outputted.