DIAGNOSIS That's the water outlet tube, and coffee oils and minerals from steam absolutely collect there. It's a maintenance spot folks often miss.
FIX 1) Unplug the machine. 2) The shower head twists off (counter-clockwise). 3) You'll now see the white silicone outlet tube. Gently pull it straight off the metal post. 4) Soak the tube and shower head in warm, soapy water or a mix of water and baking soda. Use a small brush (like a straw cleaner) to scrub the inside of the tube. Rinse thoroughly.
IF THAT DIDN'T WORK For hard scale, use a descaling solution. You can even gently pump some through the tube using a small syringe (without the needle) to direct the liquid.
HEADS UP Be gentle. The tube is sturdy but can tear if you use sharp tools inside it. Reattach it firmly before using the machine, or you'll get a leak.