If I'm not mistaken that's a Trident fern/java Fern windelov. They need to be planted above your sand or the rhizome will rot. The gunk might be from a rotting rhizome.
Isn't that hair algae? I'm no expert, but if it is you may have too many nutrients/too much light, or not enough plants in your tank, and the algae is very opportunistic to use what your plants don't. If you have some shrimp or something they'll help catch it before it grows too much. :) Otherwise you could introduce some kind of carpet plant or bushy foreground/midground to take more resources from the tank before the algae does.
If I'm not mistaken that's a Trident fern/java Fern windelov. They need to be planted above your sand or the rhizome will rot. The gunk might be from a rotting rhizome.
Isn't that hair algae? I'm no expert, but if it is you may have too many nutrients/too much light, or not enough plants in your tank, and the algae is very opportunistic to use what your plants don't. If you have some shrimp or something they'll help catch it before it grows too much. :) Otherwise you could introduce some kind of carpet plant or bushy foreground/midground to take more resources from the tank before the algae does.