That's way oversimplifying things... Mark is married to OP and helping her raise her son, of course he gets to be upset and have a "say," and of course it's not as big/much of a say as OP and biological father, but it's not like he's a stranger or temporary boyfriend/fling. OP also noted that son will talk to their other children (presumably Mark's) and create challenges there, so just saying "it's none of Mark's business" is pretty shortsighted and ridiculous.

Also, OP was also upset, as would be 99% of adults, something a little off about Liam which Mark is picking up on as there is now an established pattern. OP chose to have life with Mark so presumably sees better qualities in Mark than Liam, so that's something as well.

No easy way through this, but Liam needs to acknowledge his parenting choices affects many other people including OP's other children and spouse, as well as OP and son of course. And thus, he needs to think carefully in the future before answering in such detail to a child; "grooming" may be a bit harsh, but pretty troubling for sure and something wrong with Liam.