Chem_Geek is right about the very high chlorine levels required, if you do get mustard algae. But there are some ways to reduce that risk.

1. Using borax is probably the easiest and longest lasting, and has no undesirable side-effects. Like CYA, it's a permanent addition to the water. Unlike CYA, it is not subject to biodegradation. With a free form pool, your pool volume is only a guess, unless you actually measured it during filling, using the water meter. You'd need 20 - 25 boxes of borax + 5-7 gallons of muriatic acid, and some Lamotte borate strips (follow the link to the testkit page in my signature).

A borate level above 60 ppm reduces your pools susceptibility to algae quite a bit.

2. We don't normally recommend phosphate removal products, since if you keep your chlorine level adjusted, algae is usually not a problem regardless of phosphate levels. But, if you use them to lower your phosphates, and can avoid use of pool products containing phosphates, such as metal control products, then you can use phosphate removal products, during an algae episode, to lower levels so much that it virtually starves the algae. But, you'll need to test your fill water -- sometimes tap water has such high phosphates that this is not very practical.

3. Bromine. I'd consider this a sort of last resort. But adding sodium bromide to your pool, to produce a strong unstabilized halogen (bromine instead of chlorine) residual can also work. The downside is that your chlorine use will go WAY up during the period bromide is present, as it converts bromide to bromine. Eventually, things return to normal, since a small amount of the bromide is converted permanently to bromate on each conversion cycle, and the bromate is stable and doesn't interfere with pool chemistry.

If you choose to stay with high CYA, I'd recommend going ahead with borax. It doesn't cost that much, and gives you a margin of safety that's worthwhile. Waterbear, who has posted here for years, has found that it allows him to go on vacation for a week or so, without having to worry about algae on his return. He simply maintains chlorine levels, shocks before leaving, and then reports coming back to a clear pool. He reports that before borax, the pool was usually green on his return.