Can I use Seq with a smartphone or a tablet?

Yes, there are tons of great apps that support MIDI. With Android devices, you do not need additional equipment. Androids works as host device so you connect your smartphone directly to the USB port and you can send and receive MIDI notes. With Apple devices, you need either the inexpensive Lightning-to-USB-camera adapter or iOS MIDI interface like IK Multimedia iRig MIDI 2 and use standard MIDI din cables.