This is what all the concern is about, for those who haven't seen it.

Carl (Bumcast) displays a clump Didymo on the Southland Waiau in New Zealand. That's a Coloburiscus mayfly nymph there, hanging on the underside of the algae.

Remember, if you are fishing the South Island, immerse all your wet gear in a strong detergent after use, and dry it completely or freeze it before using it again on a different watershed.