Electrical – Solder mask expansion issue in altium

The two pads that were highlighted are probably just the first ones that showed up in their rule-checker. If you changed just those pads and sent it back to them, they'd probably come back asking about the next two pads, etc. How wide are the current pads, and how wide are the component pins? It looks like a QFP package, so I wouldn't recommend removing the soldermask. That greatly increases the chances of solder bridges between the pins, especially when hand-soldering. I would say your best bet would be to decrease the width of the pads slightly (~.02mm total, or .01mm on each side) so that they can bridge the gap with soldermask. However, if the pins of your part are too wide to fit on narrower pads, you may not have much of a choice. Either eliminate the soldermask webs between pins and just be VERY careful about solder bridges, or go to a different manufacturer who can handle soldermask webs between pads that are 0.23mm apart or less.

Use the IPC wizard to design the footprints from the device dimensions and tolerances. QFN and discretes are definitely covered, DFN may not be, at least in older versions.

You should take into account the registration and minimum solder mask sliver your PCB house can handle, but normally 75um and a 75um sliver is okay. 50um seems rather tight. For pitches finer than 500um (as with your 400um fine pitch chip) you may need to forgo the mask slivers between lands.