Within my frame care page, I mention that you need to move your seatpost once in a while. Move your seatpost!
Steel and aluminium undergo a process called galvanic corrosion that causes them to bond together.
A liberal coating of grease will stop this happening, but in the long run the grease will stop being effective, so you need to keep renewing it.
Modern dropper post mean you don’t move the post regularly. So, one a month just move your post up and down a couple of inches. Once every six months you should remove it and regrease it, sorted.
There’s a great little video about Galvanic corrosion here.
Check out the Starling frame care page here.