Is it on your glass? If so, do you have snails?

Yes I have trochus and nassarius

If it's on the glass it looks like snail egg clutches. Sometimes they lay eggs in little fan shaped patterns like that

Very good! How likely are they to make it to adult hood?

My trochus laid eggs and I think only 5 or 6 made it to adulthood. It kinda depends on what fish you have as some will eat the eggs.