Those are most likely a type of planaria. Pure protein and your fish should love them. They usually show up when we've been overfeeding a little and the substrate needs a deep cleaning to get excess food and waste out We've all had them at one time or the other. They are completely harmless but a sign to watch our feeding and tank maintenance. Hope that helps
Sorry you're having to deal with new unwanted critters in your tank! I agree with Carol they are more than likely Planaria. Over feeding and substrate issues are the cause. Although the fish may love them, you don't want them to get out of control in your tank. When the Planaria start getting over populated they can irritate the fish.
It may take a good bit of large water changes / gravel vacuums to get rid of them.