Yup, too cool mold. It's a common rookie mistake, they're afraid of getting things too hot.
I did NOT say the lead is too cool/cold. Don't make another rookie mistake, crank the heat up on the pot. That "burns" off the tin and results in no gain in clean cast bullets. Then when the mold catches up with the lead, you quickly get into the frosted bullet area, you'll be back here asking "are these frosted too bad to use?"
Get a lead thermometer. THEN you'll know what temp you're getting. Lead/tin/antimony alloy doesn't need much over 700 degrees to cast good bullets,,--IF your mold is up to temp.