This will work with the gnu version of sed ( by default on every linux )
echo -n '#HelloMyName@#IsAdam@#NiceToMeetYou@' | sed 's/#\([^@]*\)@/\1\n/g'
give me
HelloMyName
IsAdam
NiceToMeetYou
On macos
echo -n '#HelloMyName@#IsAdam@#NiceToMeetYou@' | sed 's/#\([^@]*\)@/\1\'$'\n''/g'
These are examples with echo the same will work files .
echo -n '#HelloMyName@#IsAdam@#NiceToMeetYou@'  > input.txt
sed 's/#\([^@]*\)@/\1\n/g' input.txt > sequence.txt