Beginner - Algae & dirty gravel?

Hi! I bought this fish tank about 3 months ago (38 litre). I have had fish in there for roughly 2 months as I ensured I cycled it well beforehand. The past month I’ve had this algae looking stuff on the driftwood and also the Java fern (but mainly the driftwood). My gravel is also quite brown even though I do water changes weekly and try to clean the gravel with a siphon. It doesn’t seem to do a great job if I’m honest so I might look at getting a better one as it was only cheap.

Can anyone recommend a way to clean the gravel please?

Also is the algae harmful, do I need to get rid of it? Is my filter not working well enough and that’s why my tank gets dirty so quickly?

The aquarium shop I bought the fish from said 6 Pygmy’s and 10 Tetra was a good amount of fish for my tank, so if overpopulation is the cause that makes me sad as I love my fish

Get a basic model gravel cleaner like the one in the following link and use it each week when you do a water change. the coloured gravel will get mixed up with the sand but at least the sand will be clean.

Taper your light hours back. If you're running your lights 9hrs a day taper them back to 8. Less light time will reduce algae growth