They are diatoms and very common in many new setups. They are caused by excess silicates. Eventually they will go away on their own. However, you can/should wipe them off of plant leaves where possible. Water changes should help. Some people will tell you to reduce the lighting period, but that did not make a difference for me. Otocinclus will eat it.
For me, they got worse before they got better. It was definitely frustrating but there is a light at the end of the tunnel.
Tank Journal -
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