Greenmail is a payoff to the potential acquirer to terminate the hostile takeover attempt. (from Schweser)

But will it attracts more acquirers? (because acquirer can easily get the money)

I dont think it will attracts more acquires, as this is valuable to target company, as compared to other acquirer, see its like Blackmail, where you need to pay more money, so here also you can re-control your firm by paying them higher price then price at which they acquire.

But other acquires can still see this value and try to get it for free? Like robbery?

I suspect that other (potential) acquirers will consider other aspects more important than the possibility of a greenmail payday. Assuming that they even know about the greenmail. (It’s unlikely that anybody’s going to alert the wire services about the greenmail.)

Here’s my take:

If you are acquirer seeking for synergies, greenmail is not your major consideration.

If you are a corporate raider looking for “undervalued stock” and want to earn profits through greenmails, you probably want some other fresh meat instead the one that has been robbed. Purchasing enough shares to make a takeover threat is cost intensive and time consuming, better to make sure the target is rich enough to buy back your shares with premium. Also, the target may have more pre-offer takeover defense in place after the initial fight .

Thanks! It makes sense and very helpful!