The brown stuff on top of your melt is dross. The tin is drossing out, because you are melting too hot. At 900F most of the tin in a 5% tin alloy will dross off in thirty minutes. Be aware that Lee melters have a thermostat but nothing to tell you what the temperature is. I have tested three of them with a thermometer of proven accuracy, and at their mximum settings the temp is up in the 900 region. 900 is in the beginning of the area of fuming, and lead fumes are colorless and odorless.
I like and recommend Lee melters, but only with a separate thermometer like those offeed by RCBS and Lyman. I also suspect that you have too much tin (bar solder) and that your alloy i too soft (no antimony). Tin does little to harden lead, antimony is the hardening agent.
Bullets weighing more or less than nominal weight means nothing. In fact 123.8 from a 124 grain mould is amazingly close. Because of manufacturing tolerances there can be considerable variance in moulds. In fact you can assume that of ten identical moulds you will get at least two grains variance, and it will be unlikely that more than any two moulds will cast the same weight. I have two identical Lyman moulds that cast four grains different.